Jacqueline Shipe
Hone in the Range: Lycoming Oil Pressure

Hone in the Range: Lycoming Oil Pressure

Engine oil provides lubrication and cooling for an aircraft’s engine. Ensuring your oil pressure remains “in the green” is one of the most important things you can do for your engine’s health and longevity. 

 

Oil pressure in an engine is like blood pressure in a human. Both are important indicators of internal health, and both should be kept within proper parameters to ensure longevity.

Operating pressure

The normal oil pressure range for most Lycoming engines is between 60 to 90 pounds per square inch (psi). This range is indicated by the green arc on the oil pressure gauge. The maximum oil pressure allowed for short durations is 115 psi on most models. The maximum allowable pressure has increased over the years from 100 to 115 psi. The top red line on most oil pressure gauges is 100 psi. The lowest allowable limit for oil pressure with the engine operating at idle with hot oil is 25 psi, which is indicated by the lower red line on most oil pressure gauges.

Lycoming generally sets the operating pressures for cruise rpm on their factory-rebuilt engines to between 75 to 85 psi. Most new, rebuilt or overhauled engines require a slight adjustment of the oil pressure to finalize the setting once the engine break-in process is complete. 

 

Oil flow through a typical Lycoming engine

Lycoming engines use a “wet sump” oil system. This simply means that the oil sump is mounted under the engine and oil flows by means of gravity back to the sump after it has been pumped through the engine. The sump is completely open on the top so that all areas of the engine can drain back into it, and it functions like a large drain pan. “Dry sump” systems have a separate dedicated oil tank. Oil is routed to the tank once it has completed its course through the engine. 

The Lycoming oil pump is located in the accessory housing. It consists of an aluminum outer body and two steel impellers, one of which is gear-driven off the crankshaft. (See photos 01 and 02, this page and photo 03, page 35.) It produces oil pressure in direct proportion to how fast the gears spin. At higher engine rpm, the pump produces more oil pressure than at low engine rpm. 

Oil is drawn up through the suction screen in the sump and through the oil pump impellers. The oil is then routed to the thermostatic bypass valve (also called a vernatherm valve). 

Oil continues to flow to the oil filter adapter on the accessory case and through the oil filter (or screen if the engine is not equipped with an oil filter). From the filter, oil is routed to the oil pressure relief valve. The oil pressure relief valve is located on the top right side of the crankcase. It relieves excessive oil pressure by opening a drain port to the sump to bypass some of the oil flow if oil pressure gets too high. 

Oil then travels to the crankshaft bearings and through predrilled passageways in the case to lubricate the internal engine parts through either pressure or splash lubrication. After completing its course, the oil drains back to the sump.

 

Thermostatic bypass valve

The thermostatic bypass valve is similar to a thermostat in an automotive engine cooling system. (See photo 04, page 35.) The valve remains open when the oil is below 180 F, allowing the oil to bypass the passage to the oil cooler. As the oil heats up past 180 F, the vernatherm expands and eventually contacts its seat, forcing oil to pass through the oil cooler.

An engine that has abnormally high oil temperature may have a thermostatic bypass valve that is not expanding as it should with increased temperature, or that is not seating properly due to a worn seat. The valve seat wears over time and typically gets a worn groove that gets slightly worse every time it closes. If the valve gets excessively worn it allows some oil to bypass the oil cooler even when the oil is hot. (See photo 05, page 35.) Some of the older bypass valves had retaining nuts that were improperly crimped during manufacture. Lycoming issued Mandatory Service Bulletin 518C that contained instructions for performing a heat treatment using a special Loctite to permanently secure the nuts in place. Valves that have had the Loctite treatment are typically inscribed with an “L” near the part number to indicate they have been repaired. 

As of August 2016, Lycoming no longer recommends this repair. Mandatory Service Bulletin 518D supersedes 518C and states that valve repair/rework is no longer allowed. Older-style valves with loose crimp nuts should be replaced.

Engines that suddenly develop an oil temperature problem may have one of the older-style valves with an improperly crimped nut that has come completely loose. Lycoming Service Instruction 1565 provides the procedure for replacement.

 

Oil pressure relief valve

The oil pump is a direct drive pump. This means that the pump impellers spin in direct relation to engine speed and produce oil pressure that also varies directly with engine speed. 

At high engine rpm, the pump produces far more pressure than the engine is designed to handle. Therefore, a pressure regulator must be incorporated into the system to keep pressures high enough at low engine speeds to protect the bearings and low enough at high engine speeds to prevent rupturing or damaging any of the engine components. 

The oil pressure relief valve (or oil pressure regulator) is located on the top right side of the crankcase; behind the number three or the number five cylinder, depending on whether it’s a four- or six-cylinder engine. (See photo 06, page 36.)

The oil pressure relief valve is very basic in its method of relieving excessive oil pressure. It consists of an aluminum housing with a strong spring, which presses against a steel ball. The spring keeps the ball seated. 

As oil pressure builds beyond the amount the spring is adjusted to maintain, the ball is forced off its seat by the excessive pressure. This exposes a passageway (bypass) that directs excess oil back to the sump, relieving some of the oil pressure. 

There are three types of housings. The latest style has an adjustable spring seat that can be cranked in or out as needed by means of an attached castellated nut on the end of the shaft. 

The older styles were adjusted by removing the housing and spring and adding or subtracting washers behind the spring to increase or decrease pressure. (See photos 07 and 08 on page 36 and photo 09 on page 38.)

The oldest style housing was short and had an adjustment of zero to three washers maximum. (See photo 10, page 38.) The longer housing allowed up to nine washers maximum to increase spring tension. (See photo 11, page 38.) Each added washer increases oil pressure approximately 5 psi. On the externally adjustable models, one turn in (clockwise) increases oil pressure approximately 5 psi. 

There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators. (See photo 12, page 40.)

One of the more common problems with the oil pressure regulators is with the seat that the steel ball contacts every time it closes. The seat is simply a machined aluminum section of the crankcase itself on most engine models, and over time it can become worn, especially if the ball is not contacting the seat dead in the center. 

If oil pressure varies excessively with engine rpm, especially at lower engine speeds, the regulator ball and seat may not be closing properly. Poor contact allows some of the oil to bypass back to the sump when it shouldn’t. (See photos 13, 14 and 15 on pages 40 and 42.)

If the cast aluminum seat has an irregular wear pattern in it, Lycoming recommends rigging up a makeshift tool out of an old ball welded to a steel rod that is thick enough to be struck with a hammer, then inserting the newly made tool squarely against the seat and giving it a couple of sharp hammer strikes to reform the seat, allowing a tighter fit between a new ball and the seat. 

The field method of repairing a worn or non-concentric seat that most mechanics employ is to use the same tool mentioned above, but instead of striking it with a hammer, they use a tiny bit of valve grinding compound on the ball to re-lap the seat. Care must be taken to prevent the compound from getting into any of the oil passageways during the process, but overall this method tends to work well to reform the seat and regain a good seal between the ball and seat. (See photo 16, page 42.)

Some of the earlier engines did have a replaceable seat insert that could be changed out and replaced if it was worn, but the most common seat is the cast aluminum type mentioned above. 

 

Oil pressure gauge

The oil pressure gauge on many airplane models consists of a mechanically-actuated “Bourdon tube.” The Bourdon tube is a somewhat rigid, coiled, hollow tube. 

The tube is connected to a small oil pressure line and as oil pressure increases, the tube is stretched to a straighter, uncoiled position. The amount that it stretches varies directly with the pressure. An attached needle and gear mechanism allows the varying pressure to be read on the oil pressure gauge. 

These mechanisms can get dirty and stick, or the gearing mechanism can get worn and not indicate correctly. A shaky needle is often caused by a worn gear mechanism in the gauge.

Some aircraft use an oil pressure transducer or sending unit that looks similar to the oil pressure switch used for Hobbs meter installations. It is a unit that has an oil pressure line piped into one side, and electrical wires connected to the other side. Pressure is converted to an electrical signal and wires are run to a gauge that displays the oil pressure reading.

The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading. 

 

Troubleshooting oil pressure problems

Most oil pressure problems can be adjusted back to normal with the regulator or traced to a malfunctioning regulator or gauge. Sometimes, the trouble is a little more difficult to repair. 

The first step in correcting abnormally high or low oil pressure should be to double-check the pressure reading with a separate pressure gauge to confirm that the oil pressure really is too high or low. 

Check the oil temperature, too. Low oil pressures will produce increased oil temperatures, and vice versa; overly high temperatures thin the oil and can cause a lower-than-normal oil pressure reading.

Excessive internal engine clearances due to excessive wear or a bearing failure can become so great that the output of the pump is insufficient to fully pressurize the oil system. This is typically a worst-case scenario and lower oil pressure readings occur gradually over time. 

Excessive oil pump clearance between the impellers and the housing can also cause degraded oil pressure output.

Oil viscosity plays a role in oil pressure as well. A slightly lower than normal oil pressure may be caused by using too thin an oil depending on where the plane is operated. 

A clogged suction screen or partially blocked passage between the screen and pump can also cause low oil pressure.

A higher-than-normal oil pressure reading, especially one that occurs suddenly, can be indicative of a blockage somewhere in the system, usually downstream of the pump. Conclusion

Oil pressure readings should be consistently monitored so that any deviation from normal operation can be detected and remedied quickly. Consistent, normal oil pressure from startup to shutdown helps assure that an engine will run reliably for a long time.

 

Know your FAR/AIM and check with your mechanic before starting any work.  

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to . 

Resources

Lycoming Mandatory Service Bulletin 518D http://www.lycoming.com/node/15796

 

Lycoming Service Instruction No. 1565A http://www.lycoming.com/content/service-instruction-no-1565-a

Camshaft & Lifter Wear

Camshaft & Lifter Wear

Corrosion attacks camshafts and lifters before most other bottom-end engine parts, especially when an aircraft doesn’t fly frequently. Learn how to prevent, identify and treat common camshaft and lifter problems before they cost you!

Many folks think of engine wear and tear and overall condition as being represented by the amount of operating time since the last overhaul. The “time since major overhaul,” or TSMOH for short, is typically entered by mechanics into an aircraft’s logbooks at every major inspection and annual. One of the first questions that buyers ask when considering an aircraft for purchase is “How much time is on the engine?”

Cumulative engine operating time does matter. Moving parts, particularly in the high heat and high stress cylinder environment, do wear with use. However, one of the main reasons that engines have to get torn down and rebuilt isn’t due to wear from operating regularly, but from corrosion that occurs during prolonged periods of non-use. 

An engine’s camshaft and lifters are particularly susceptible to damage from corrosion. Corrosion and the resulting wear on the camshaft and lifter bodies are one of the primary reasons engines get torn down before reaching the manufacturer’s recommended time between overhauls (TBO).

Lycoming crankshaft and camshaft gears as viewed from the accessory case section (rear) of the engine.
A Lycoming case half with camshaft installed, viewed from the rear.
Camshaft construction and function

Camshafts are a simple-looking part, but they play a major role in engine operation because they control and drive the entire valve train. The camshaft is gear-driven and typically turns at one-half the speed of the crankshaft. 

Camshafts are made of carbon steel and have varying numbers of “lobes” along their lengths. Each lobe has an elliptical shape and is put through a hardening process called “carburization” during its manufacture. 

The carburization process involves heating the camshaft in a specially-designed furnace or oven and exposing it to carbon monoxide gas as it is heated. This process causes the exposed surfaces on the camshaft (primarily the lobes) to absorb extra carbon, which makes the surface of the lobes very hard. The depth of the hardened layer on the lobe is about .015 inch (fifteen-thousandths
of an inch). 

The hydraulic lifter units from a Continental C-85 engine.
The valve train components: camshaft, lifter, pushrod, rocker arm and valve.

A solid Lycoming O-235 lifter.
Lifter construction and function

The lifter consists of a cast-iron outer body that houses a hydraulic unit. There are a couple of Lycoming engines that still utilize solid lifters with no hydraulic unit, but the vast majority of engines have hydraulic lifters. Most mechanics use the term “lifter” when referring to both the lifter body and the hydraulic unit inside the body. 

Lifters are also sometimes referred to as “tappets.” The face of the lifter body is the part that is in continuous contact with the camshaft lobe that it rides on. The lifter face is made of chilled cast iron which gives it increased wear resistance. It is tough, but not nearly as hard as the carburized surface on the cam lobe. 

The lifter is designed to be a little softer than the lobe, so that the two are compatible with each other. If deterioration does start to occur, it generally occurs first on the lifter rather than the camshaft lobe. 

The hydraulic unit in the lifter body is designed to act as a solid unit as it opens the valve. It must also be able to expand or contract as needed to take up all extra clearance in the valve train. There are some variations in the exact design, but the basic function is the same. 

The interface between the camshaft lobe and lifter body is under very high pressure and is unforgiving of any defects.
A corroded camshaft lobe.
A Continental O-470R camshaft shown installed just above the sump. The lobes on this camshaft all exhibited normal wear patterns; engine was torn down due to time. not corrosion.
Valve operation

The cylinder valves are linked to the camshaft through the lifter, push rod and rocker arm. The camshaft rotates and the top of the lobe raises the lifter to its highest point. The push rod in turn raises the rocker arm end to which it is joined.

Since the rocker arm is mounted near its center on a steel shaft, it acts as a fulcrum and raising one end causes the other end to lower. The opposite end of the rocker arm that is in contact with the top of the valve stem then lowers the valve to its full open position. 

The time that the valve opens in relation to all other moving engine components, and the length of time that it stays open are both determined by the height and position of the camshaft lobe. 

The valve clearance, or valve “lash” as it is often called, is measured with both the exhaust and intake valves closed with the piston at top dead center (TDC) on its compression stroke. The clearance is measured with the hydraulic unit of the lifter “bled down,” or in its flattest position with no oil in the reservoir of the lifter. 

The clearance is checked between the valve stem and rocker arm with the push rod socket on the rocker arm pressed in toward the camshaft. The engine manufacturers set a minimum and maximum clearance for the valves. Too much or too little clearance can cause increased wear on the valve operating mechanism. 

The clearance is adjusted on engines with solid lifters by means of an adjustment screw on the rocker arm itself. On engines with hydraulic lifters, the adjustment is made by replacing the push rod with another of a different length. 

The valves are held in the closed position by multiple strong valve springs. The camshaft and lifter must overcome the valve spring tension every time the valve is opened. This generates a large amount of pressure between the lifter face and the camshaft lobe. 

A Lycoming O-235 case half with the camshaft in place (above) and with the camshaft removed (below) to show where it rotates.
A Lycoming O-235 case half with the camshaft in place (above, left) and with the camshaft removed (above, right) to show where it rotates.
A severely spalled lifter face.
Lifter spalling

The camshaft and lifter bodies typically are lubricated in most engines by what is called “splash lubrication.” There are no pressurized oil ports on standard engines that deliver oil to the cam and lifter surface; they run only in the oil that is thrown off the crankshaft and rods as they rotate. 

At engine start-up, it typically takes a few revolutions of the crankshaft to begin getting any significant amount of oil to the camshaft. The lack of pressure lubrication along with the high pressure between the interface of the camshaft lobe and lifter face create an environment that is unforgiving of any defects in the surfaces of either the lobe or lifter. 

Engines that sit dormant for extended periods of time (between two to six months, depending on the environment) are subject to developing corrosion on both the lifters and on the cam lobes themselves. 

The camshaft on Lycoming engines is located in the top of the crankcase above the crankshaft. Its position in the top of the case means that oil drains off it first once the engine is shut down. On Continentals, the camshaft is located beneath the crankshaft, but it is still above the oil sump and residual oil eventually drains off it as well. 

The crankshaft is not as vulnerable to corrosion because its metal-to-metal contact surfaces are encased in bearings. The bearings typically hold a film of oil on the crankshaft surface. The camshaft has no bearings. It spins inside machined grooves in the crankcase. The lobes and lifters have a lot of their surfaces fully exposed to the air that drifts through the engine. 

Aircraft engine breathers are simply vents designed to prevent pressurization of the crankcase with the engine running. However, they also vent and expose the internal engine components to any humidity that may be present in the outside atmosphere as the airplane sits parked.

Condensation occasionally occurs inside the engine after shutdown as parts of the engine cool at varying rates. Condensation also occurs with different weather conditions during times of non-use. Moisture can cause small freckles of rust to form on the lifter face. As the corrosion progresses, the rust eats into the lifter surface. 

Once the engine is started again, the small patches of rusted material fall out as the lifter face is pressed against the camshaft lobe. After this process starts, the constant pressure between the cam lobe and lifter wears off more and more material even though the engine is being lubricated during operation. This process is referred to as “spalling” of the lifter. 

The removed material and the rough lifter face will eventually wear the camshaft lobe. If lifter spalling is caught early, the problem may be fixed by replacing the affected lifters. However, if any of the camshaft lobes have begun to wear, the entire camshaft—in addition to the damaged lifters—must be replaced. 

A lobe on a Lycoming O-235 camshaft that has been damaged from a spalled lifter.
The spalled and corroded lifter face.
Lifter inspection on Continental engines

Most Continental engines use cylindrically-shaped lifter bodies that can be removed without splitting the crankcase. Lycoming engines use mushroom-shaped lifter bodies that have a larger diameter at the face of the lifter. These types of lifter bodies cannot be removed without splitting the case. 

On the larger Continentals (O-470 series, IO-520, etc.), lifter removal only requires removal of the affected cylinder valve cover, rocker arm, push rod and the push rod housing. The housing is removed by compressing the tension spring on the inboard part of the housing to release it. 

Once the housing is out of the way, the lifter can be removed by gently using a pick to grab it by the oil galley in the push rod socket and sliding it out. Engine manufacturers do not recommend using a mechanic’s magnet to remove a lifter because it could magnetize the ball in the check valve of the hydraulic unit and cause the valve to malfunction.

After the lifter has been removed, the camshaft lobe can be inspected using a bright light. Continental Service Information Directive SID 05-1B contains inspection criteria which should be used to determine if a camshaft is still airworthy. Continental recommends using a mechanic’s pick to probe any surface deformities. 

If the pick catches, the camshaft most likely will need to be replaced. If the camshaft lobe looks OK with no surface damage, the lifter itself can be replaced with a new one if it shows any sign of wear. 

Some mechanics are leery of replacing lifters without replacing the camshaft because the lifters and cam lobes do wear into each other slightly during the initial engine break in, but it is primarily the lifter, not the camshaft lobe that wears. 

Aircraft owners who have large Continental engines that have been dormant for an extended period of time would be wise to inspect their lifters before they run the engine again. The process of doing so may very well catch a corroded lifter before it has a chance to wear a cam lobe. Six months would be considered an extended period of time under usual conditions; two months in humid or salty areas.

A Continental O-470 cylinder with one of the rocker arms and pushrods removed and the push rod housing partially removed.
Clues that a camshaft may be wearing

Most camshaft and lifter wear goes undetected until fragments or small magnetic filings show up in the oil filter during an oil change. Even though amounts may be very small, the camshaft and lifter bodies are usually the culprits if an engine suddenly starts making magnetic material. Steel cylinder barrels or steel cylinder rings also are prone to rusting a little, but they generally stop shedding any rusted debris as soon as the airplane is flown a time or two. 

Regularly-occurring magnetic filings are most likely coming from the lifter bodies, and once lifters start shedding surface material, they continue to degrade and wear with subsequent use. The good thing is that other than possibly causing a tiny bit of engine roughness, cam lobe and lifter wear is not something that causes sudden stoppage of an engine. 

If small amounts of filings are found, the oil filter element and its contents should be sent to an aviation oil analysis lab to determine the source. An oil-only analysis will probably show nothing unusual in these cases because when the lifters start spalling, the flakes and filings they shed are so large that they get caught in the filter rather than being suspended in the oil itself. Most labs can examine the filter element itself to determine where the metal is coming from. 

Some mechanics that find very small amounts of magnetic material (less than enough to cover the end of a mechanic’s magnet) will recommend flying the plane for another 10 hours or so, then checking the filter again. The hope is that it will stop on its own, but an engine that is past the break-in period should not be making magnetic material. 

If the filings are coming from rusted cylinder barrels, an owner might get lucky and the oil filter might be clean on the next check, but it’s been my experience that almost every time I’ve found magnetic debris in the filter, it only gets worse and worse with use. 

An O-470 cylinder with the push rod housing; the housing is removed by releasing the spring tension.
A severely worn camshaft lobe.
A Continental IO-520 with the cylinders removed. The lifter inspection is easy with the cylinders off because the lifters can simply be slid out.
An O-470 push rod and shroud.
Reground versus new camshafts

A camshaft or lifters that are worn badly enough to necessitate an engine teardown will require replacement. Many engine rebuilders in the field have no problems using a reground camshaft (one that was still within serviceable limits that has the lobes re-machined to a smooth finish.) Lifter body faces can also be reground to a smooth finish. 

If an owner chooses to use a reground camshaft or lifters, they should only get them from a high-quality aircraft machine shop experienced in the regrinding process. The carburized hardened layer on the camshaft lobes is only around .015 inch deep, and the layer’s depth can vary a little. 

If a machine shop accidentally gets below the carburized layer at any time during the regrinding process, the camshaft lobe will wear down rapidly once it is put back into service. The initial hardening process of carburization is completed only at manufacture. It can’t be duplicated in the field because it is almost impossible to keep the camshaft from warping during the carburization process. 

Additionally, the apex (or top) of the camshaft lobe is tapered slightly. One side of the lobe is slightly (around .003 inch) higher than the other side. This design causes the lifter body to spin as it contacts the lobe. This prevents the camshaft lobe from contacting the lifter face in the same place every time, and helps prevent wear on the lifter face. The exact taper may be hard for some shops to duplicate.

I personally would rather have a new camshaft and lifters if the engine is being torn down to the extent that they were accessible. However, most reputable engine builders that I spoke with had no problem using reground cams and lifters, depending on where they were done, and I know of more than a few engines that have easily made and even gone past TBO with reground parts.

A Continental IO-520 with the cylinders removed. The lifter inspection is easy with the cylinders off because the lifters can simply be slid out.
A severely spalled lifter out of this engine; it ruined the camshaft lobe it was riding on.
Prevention

The best thing an owner can do to prevent camshaft and lifter problems is to regularly fly his or her plane—at least once a week in humid climates. Each flight should be long enough to get the oil temperature up to at least 180 degrees to evaporate any water in the oil system. Changing the oil regularly and often is a good preventative maintenance practice. The addition of an anti-corrosion additive also helps. 

The initial cost of prevention generally is greatly rewarded with a long-lasting healthy engine. 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Resources

TCM Service Information Directive 05-01B, “Inspection Guidelines for CM Camshafts and Hydraulic Lifters”
http://www.tcmlink.com/pdf2/SID05-1B.pdf 

Anti-corrosion additives
AvBlend – CFA supporter

ASL Camguard
aslcamguard.com

Oil Changes: The Original DIY

Oil Changes: The Original DIY

A&P Jacqueline Shipe details the process of changing the oil and filter on an aircraft in this fifth installment in Cessna Flyer’s DIY series.

 

Most planes have a drain valve to facilitate oil changes. There are two primary types of valves: the style that pushes straight up to lock in place, and the type that has to be pushed up and turned.

One of the items that is labeled as preventive maintenance by the FAA that a pilot may perform on his or her own airplane is the cleaning or replacing fuel and oil strainers or filter elements.

An oil change performed at a regular interval is one of the best things that can be done to prolong engine life. Clean oil lubricates and carries away harmful deposits better than dirty oil; plus, inspecting the contents of the removed filter or screen often helps to detect a malfunction before it becomes catastrophic. 


Draining the oil

Warm oil drains faster than cold oil, so it is nice to run the engine a little before beginning the oil change to cut down on the amount of time it takes to drain. (If you choose to do this, remember that the exhaust components will be especially hot. Use care.) 

The first step in the oil change process is draining the old oil. The container the oil is being drained into needs to be large enough to accommodate it. An old five-gallon bucket works well. 

Most planes have a drain valve on the bottom of the sump to facilitate oil changes. There are two primary types of valves: the style that pushes straight up to lock in place, and the type that has to be pushed up and turned. 

If it is possible to reach the valve through an opening in the lower cowling, an old rubber hose that fits snugly over the drain end of the valve works great to port the old oil into a bucket and saves having to remove the entire cowling. (It makes a huge mess if the hose pops off the drain valve midstream, so be sure it is secure.)

If the valve is inaccessible, the lower cowling will have to be removed, which isn’t such a bad thing because it allows access to give the engine a good looking-over. 

The space between the filter and the other parts on the accessory case is very limited on some engines, like the one above; a short wrench may be required in these situations.
This black rubber drain hose is inserted through the cowl flap onto the drain valve on the sump in order to port the old oil into a bucket.
Removing the filter

Filter removal begins with cutting the old safety wire. The wire should be cut in the loop that goes through the safety hole on the engine, never pulled through. 

If the wire is pulled, it will cut through the soft, very thin tab on the engine and the tab will then be useless. Once this happens, the safety wire has to be attached at another point on the engine. 

After the safety wire is removed, the filter is loosened by using a one-inch size wrench on the end adapter to remove the filter. It is best if the wrench is the six-point style because the adapters are fairly thin; if the filter is stuck on the engine from being overtightened the time before, a lot of torque will be required to break it loose. The ears on the adapter can be easily rounded off if this happens. 

The space between the filter and the other parts on the accessory case is very limited on some engines; a short wrench may be required in these situations. 

The filter usually drains a little oil as it is removed. An empty oil container turned sideways with the top cut off can catch any oil that dribbles out as the filter comes off. Once the old filter is off, it can be placed aside and allowed to drain.

This filter has been cut open, and the inner part is being lifted out of the housing. The paper pleats in the inner part have to be cut out of the inner metal housing with a utility knife and spread apart to be thoroughly inspected.
Installing the new filter

The new filter needs to be double-checked to be sure it has the correct
part number. 

Some folks jot the tail number and tach time on the side of the filter using a permanent marker when it’s installed. This helps to determine when the oil was last changed without having to drag out the logbooks. It also ensures that the old filter won’t get mixed up with one that came off another engine or airplane. 

The new filter also needs to be inspected to be sure there are no leftover pieces of packaging or debris laying in it. If the filter has any dents or other signs of mishandling, it should be discarded or returned. 

After the filter is inspected, the gasket should be lubricated to protect it during installation. Some mechanics also partially fill the filter with clean oil before it is installed to help prevent a dry startup. 

Dow Corning DC-4 is the lubricant most filter companies recommend for greasing the gasket on the filter prior to installation. The lube helps with removal the next time, but mainly it keeps the rubber seal from being broken loose from the filter during installation. It usually costs around twenty bucks for a tube of this lubricant, and one tube can last for several years.

The oil filter adapters are designed to provide a place to grab the filter with a wrench and are spot-welded on during the manufacturing process. If the new filter is overtightened during installation, one or more of these spot welds may break loose, which can cause a severe leak initially or later on down the road. 

Some mechanics never use the wrench adapter to install a filter because of this. At any rate, care should be taken to not overtorque the new filter. 

Once the new filter has been installed and properly tightened, it is ready for safety wire. Wherever the new wire is routed, it should be situated so that it can’t chafe into the filter or into anything else. Some mechanics slip a piece of heat shrink or something similar over the new wire to help prevent it from rubbing into the filter. 

The wire should have a positive pull on the filter and be fairly taut. 

An oil screen screen is safety-wired in the same manner as a filter. The screen shown is on a Continental C-85 engine; the oil temperature probe screws into the back.
Shown above are the new and old style piston pin plugs. The top one has the plugs that screw into the pin, while the old style (bottom half of photo) is the type that has the plugs that butt up next to the pin. All three pieces of the old style plug are supposed to be able to slide from side to side, and are contained by the cylinder wall. The plugs on this old style pin can sometimes develop significant wear in a short time if they get hung in a fixed position.
Service the oil screen

Oil screens are removed in a similar fashion as an oil filter. The screen is small (compared to a filter) and generally has a larger area for access. The screen is safety-wired in the same manner as a filter. 

A copper crush washer is used as the seal on the oil screen housing, and the crush washer should be replaced at each oil change. 

Engines that didn’t originally come with an oil filter as standard equipment and have an aftermarket oil filter kit installed often will have the original screen as part of the oil system. 

The oil screen should be regularly removed and cleaned in addition to replacing the filter on these engines because the dirty oil goes through the screen before it reaches the filter. If it’s neglected, the screen can actually become clogged with debris and restrict oil flow to the engine.

This severely worn piston pin plug (left) has almost completely disintegrated. A standard piston pin plug has been placed on the right for comparison.
Perform a leak check

Changing the oil and filter is fairly straightforward, but there are some precautions to take. The oil pressure at the point where it flows through the filter or screen is very high; a leak in a gasket can quickly lower the oil level if the airplane is flown with a leaking filter or crush washer. 

Any time the oil filter is replaced or the oil screen is cleaned, the engine should be run on the ground afterward and then checked for leaks. After the proper oil amount is added, the engine is ready for a ground run and leak check. 

Average wear of an old style piston pin plug will look similar to what you see above.
Inspecting the oil filter and/or screen

The contents of a removed screen or old filter should always be thoroughly inspected for any metal. A clean white paper towel works well to catch the residue off a screen as it is cleaned. Most mechanics use a solvent or parts cleaner to flush a screen. 

An old filter should be cut open using a filter cutter so the inner part of the filter can be removed. The paper element needs to be completely cut out of the inner metal housing and unfolded so it can be inspected. 

When the pleats are still wet with oil it can be hard to see very small particles embedded in the paper. Some folks let the filter drain overnight because of this; the element can be folded sideways and compressed in a vice if a person is in more of a hurry. A vice will squeeze out the old oil but leave any contaminants in the pleats. 

A magnet gently drug across the filter pleats or the paper towel will pick up any steel particles. Finding any metal is cause for concern, but finding steel contaminants is a major concern. The source of the steel should be tracked down and remedied if there is more than a trace amount. 

Aluminum flakes or slivers are generally caused by wear on one or more of the piston pin plugs in the cylinders. These plugs are made of aluminum in most engines; some are now made of brass. 

They contain the free-floating piston pins on either side and keep them from contacting the cylinder walls. Aluminum and brass are relatively soft compared to steel and are designed to wear without damaging the cylinder wall. 

If just a few particles of aluminum or brass are found, it is best to change the oil at an earlier interval next time in order to inspect the filter contents. Generally the problem goes away on its own because the plug wears itself in a little and stops, but not always. 

Increasing amounts, or large initial amounts, of any metal is grounds for disassembling the cylinders—and possibly the entire engine—to discover the cause. It is best to let an experienced mechanic inspect oil filter contents if any metal is present to get a second opinion about what action, if any, should be taken.

Regular oil changes will save money in the long run. Owners that change the oil, filter and/or screens themselves will not only save money on labor costs, but also have a good understanding of the health of their airplane engine.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Propeller Vibration & Dynamic Balancing

Propeller Vibration & Dynamic Balancing

Feeling a bit shaky when you’re flying (and tired when you land)? It might not be your nerves; but rather the side effects of excessive vibration. Vibration can originate from an aircraft’s engine, propeller or spinner, and if unchecked, can lead to further mechanical problems. Vibration can also cause pilot and passenger fatigue. Luckily, computerized dynamic propeller balancing is a cost-effective route to help dampen those pesky vibrations.

Aircraft propellers are manufactured to be durable, strong and able to absorb the stresses of flight. Depending on the model, propeller blades weigh anywhere from a few ounces to several pounds. The rotational speeds and aerodynamic loads imposed in flight on a propeller make it susceptible to vibration if it has even the slightest imbalance.  

Excessive vibration can come from an aircraft’s engine, propeller, spinner or a combination of all three.


Why vibration matters

No matter what its source, excessive vibration can lead to a number of problems. The engine’s vibration isolators are designed to filter out most of the vibration so that it is not transmitted to the airframe, but they don’t eliminate all of it. 

An out-of-balance propeller that causes the engine to vibrate in its mount will wear out the vibration isolators. Cracks in the airframe can form as a result of excessive shaking. Cracks can also form on the cowling itself, and on the spinner or spinner bulkhead. 

Vibration can cause cracked or loose exhaust connections. As an illustration of the importance of balance when it comes to exhaust systems, the makers of PowerFlow exhausts actually require a dynamic propeller balance for their exhaust systems to qualify for an extended warranty. 

That’s not all. Vibration is hard on engine components and can cause premature engine wear. On top of creating mechanical issues, vibration is also source of pilot stress and fatigue.
Where’s it coming from?

The frequency at which a vibration is occurring gives a clue as to whether the vibration is being caused by the propeller itself or the engine. 

Vibrations are counted by the number of vibrations per revolution (of the propeller). They are referred to as “two-per,” “half-per” and so on.  A one-per vibration occurs on every revolution of the propeller and is indicative of the propeller itself, or in rare cases the engine crankshaft. A half-per vibration occurs on every other revolution of the propeller and is usually caused by a cylinder malfunction. A small vibration occurring at a frequency greater than once per revolution (two-per or more) is usually indicative of bearing wear or a malfunction in an accessory such as the alternator. 

Vertically-mounted static propeller balancer
Causes of vibration: engine and mounts

As noted above, vibration can originate from sources other than the propeller. The engine and engine mounts can be a culprit.

Compression imbalances or a cylinder with excessively low compression can cause vibration. Extreme wear in crankshaft counterweights can also cause vibration. 

Worn engine vibration isolators permit excessive vibration, and can allow the front of the engine to sag downward.

A cracked engine mount can cause a great amount of vibration. 

Horizontally-mounted static propeller balancer
Causes of vibration: spinner

Spinners that have a heavy spot due to a manufacturing defect or repair can cause a slight imbalance which produces vibration. Laying the spinner on a flat table and slightly rolling it can sometimes detect a heavy spot on the spinner. If the spinner comes to rest with the same spot on the bottom each time it probably has a heavy spot.  A spinner with a heavy spot can make dynamically balancing a propeller difficult.

The forward tip of the spinner should be aligned with the center of rotation of the propeller. If the nose of the spinner appears to wobble when observed by an onlooker outside of the plane as the engine is run, the spinner should be realigned by loosening the mounting screws and retightening them as the spinner is held firmly in place. 

Cracked or broken spinner bulkheads can also cause the spinner to wobble. It is a good idea to inspect them closely if any defect is noted.

Static balance weights for use on a controllable-pitch propeller
Causes of vibration: propeller

Vibration originating from the propeller is usually caused by a mass imbalance. A mass imbalance is when the center of gravity of the propeller is not in the same location as the center of rotation of the propeller. It is usually caused by the removal of material on blades to repair nicks or from differing degrees of blade erosion. Luckily, this can often be remedied by balancing the propeller and checking for correct blade track and indexing.

Sensor mounted on top of cowling with a clear view of the rear of the propeller
Static propeller balancing

Aircraft propellers are statically balanced at the time of manufacture and at propeller shops. Static balancing is the process of checking the weight of the hub and blades for even distribution. This ensures that the propeller is not subjected to any turning or bending force due to a heavy area on one of the blades or hub. 

During a static balance, the propeller is mounted on a mandrel resting on low-friction bearings so that the propeller is free to spin, with a minimal amount of force needed to move it. (See photo 01, page 26.) 

When the propeller is turned slightly, it should remain in the new position it is placed in without backing up or continuing to turn. The process is similar to balancing a wheel assembly. 

If an imbalance exists in the propeller or hub, the heavy area will cause the propeller to rotate so that the heavy spot ends up on the bottom. 

Some shops mount the propeller in a horizontal plane on the top of a shaft that has an indicator rigidly suspended from the bottom of the mounting shaft. (See photo 02, page 26.) 

As the propeller is turned, if the indicator on the bottom of the shaft leans to one side rather than maintaining a vertical position, the propeller has a heavy spot. 

Weights can be added or subtracted to the hub to statically balance most controllable-pitch propellers. (See photo 03, page 26.)

Fixed-pitch propellers are statically balanced by removing an allowable amount of material from the heavy blade. 

Static balance is initially adjusted at propeller assembly and fine-tuned after the propeller is completely assembled and painted. Propellers with de-icing (“hot props”) are adjusted after all anti-ice boots are installed. 

Reflective tape placed on a propeller blade
Propeller blade track

Once a propeller has been statically balanced and installed on the aircraft, the track of each blade should be checked. The blade track refers to the path that each blade tip travels. On a perfect propeller, the tracks will be identical. 

The track is checked by placing a solid object next to a propeller blade near the end so that the propeller blade is free to rotate past it, and marking exactly where each blade tip passes the object. There shouldn’t be more than 1/16 inch in difference between the tracks. 

The airplane needs to be chocked so that it can’t move and the propeller should be pushed in slightly against the engine as each blade is checked to remove the endplay from the thrust bearing in the engine. A blade that is out of track will cause an aerodynamic imbalance because its angle of attack will differ from the other blade or blades. Also, differing blade tracks can indicate that the propeller has been damaged in some way

Analyzer (above), sensors and cables (left). Ready for a test run.
Propeller indexing

The propeller index refers to the location on the crankshaft flange where the propeller is installed. Engine and airframe manufacturers designate where the propeller should be installed on the crankshaft flange with the No. 1 cylinder’s piston on top dead center of the compression stroke. Typically, on most small, two-blade, fixed-pitch propeller aircraft, the propeller is installed with the top blade aligned with the bolt-hole preceding the vertical position as viewed facing the propeller. This corresponds roughly with the 2 o’clock and 8 o’clock position.

There’s no reason to guess as to what indexing is correct. The maintenance manual for each aircraft model gives the specifications on where to install the propeller on the flange. Propellers installed in the incorrect location on the flange can cause vibration.

Dynamic propeller balancing

Dynamic propeller balancing is the process of checking for vibration while the propeller is in motion. The propeller is installed on the engine and the engine is run through its complete rpm range. 

A dynamic balance is performed using a vibration-detecting sensor mounted to the top of the engine, and a photo sensor mounted so that it has a clear view of the rear of the propeller blades. The sensor detects a reflective piece of tape placed on the rear of one of the blades each time it passes through the sensor’s beam. (See photos 04 and 05, page 28.)

The vibration sensor is an accelerometer containing a crystal which detects the direction and amount of force of each vibration. The sensor is calibrated and reads the force in inches per second (IPS). This information, along with the location of the reflective tape provided by the photo sensor, is transmitted to the analyzer. 

The analyzer attached to both sensors gathers information about the amount and frequency of any vibration, accurately records rpm and calculates the amount and location of weight to be added to correct an imbalance. (See photo 06, page 30.)

After the engine and propeller run, the specified amount of weight is placed in the location given by the analyzer. The weight is added according to the propeller balance equipment manufacturer’s instructions. Usually, the propeller is rotated by hand so that the reflective tape is lined up with the photo sensor. The number of degrees shown on the screen of the analyzer marks the spot needing the weight. A measurement is made from the accelerometer in the direction of propeller rotation, and the spot is marked.

On airplanes with Lycoming engines, the weight is usually added to one or more of the holes on the outer section of the starter ring. On airplanes with Continental engines, the weight is usually added by drilling a hole in the spinner backing plate. An AN3 bolt or #10 structural screw with a locknut and stacks of large-area washers are used to add weight. A maximum of six washers is allowed per screw. 

It may take several runs and additions and subtractions of weight to eliminate an imbalance, or at least bring it into a reasonable range. Sometimes, especially with Lycoming engines, the weight can’t be placed at the location pinpointed by the analyzer because a hole isn’t available at that exact spot. In that case, the weight should be halved and installed at two different holes on each side of the target location. 

The installed weight(s) should be checked for adequate clearance from the starter and other components by pulling the propeller around by hand and making sure the weight doesn’t come close to contacting anything as it rotates with the propeller. 

Vibration levels are labeled according to a standard scale. Vibration levels of 0 to 0.07 IPS are considered good. 0.07 to 0.15 IPS is considered fair. 0.15 to 0.25 is considered slightly rough. 0.25 to 0.5 is moderately rough. 0.5 to 1.0 is very rough, and 1.0 to 1.25 is considered dangerous. 1.20 is the maximum allowable FAA limit for a dynamic imbalance. 

Dynamic balancing errors

The person performing a dynamic balance should use high-quality, calibrated equipment. Erroneous sensor readings will cause weight to be added in incorrect locations. 

The propeller and spinner should be clean before the balance procedure is started. Spinners should be removed and cleaned on the inside as well, especially on propellers that require grease. 

Controllable-pitch propellers should be greased and serviced properly with nitrogen as required before the balance procedure is started. 

Finally, the weather conditions should be favorable. Accurate readings are best obtained when the engine and propeller is run in calm air. The plane should be pointed into whatever wind is blowing, not only to aid in engine cooling, but also because a tailwind or crosswind can affect the readings.

Troubleshooting other problems during a dynamic balance 

The propeller balancing equipment can also be used as a troubleshooting aid when an engine does not seem to be developing its normal power.

The photo tachometer gives an accurate rpm indication at full-throttle static rpm.  If the engine is not making its full rated power, further investigation is warranted. A lower-than-normal output is indicative of an engine defect or excessive wear. 

Most airplanes use mechanical tachometers. These tachometers are seldom accurate and can read high or low when compared to actual engine rpm. The photo tachometer is more accurate for troubleshooting use. It will also help determine whether a mechanical tach is reading incorrectly; and if so, by how much.

Conclusion

Most airplanes have at least a slight propeller imbalance, even if it hasn’t become bad enough to be noticed by the pilot. It is always best to correct vibration problems early because they tend to grow in magnitude as wear occurs. 

The benefits of propeller balancing greatly offset the cost. Reducing vibration helps reduce wear and fatigue, extending the service life of many components not only on the engine, but on the airframe itself. 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Resources

PowerFlow exhausts
PowerFlow Systems, Inc.
powerflowsystems.com

Exhaust System 101: Inspection & Maintenance

Exhaust System 101: Inspection & Maintenance

Thin metals and fluctuating temperatures can literally “exhaust” your airplane’s exhaust system. Read on to learn how an aircraft exhaust system is constructed and get several practical tips to identify common trouble spots and prevent unnecessary damage.

Aircraft exhaust systems require detailed inspections and regular maintenance because of the highly corrosive and hot environment in which they operate. Cracks and leaking connections can cause significant amounts of damage to other engine parts, as well as possibly exposing airplane occupants to carbon monoxide. Understanding how to promptly identify exhaust leaks and repair them correctly can help owners and mechanics avoid expensive and/or dangerous problems down the road. (While owners can perform several of the visual inspections outlined in this article, consult your mechanic before performing any disassembly, assembly or repair. —Ed.)

Exhaust system anatomy and construction

Most exhaust risers and mufflers are made of stainless steel or Inconel. (For those who aren’t familiar, Inconel is a trademarked name for a superalloy used in high temperature applications. —Ed.) Some older exhaust systems also contained parts that were made of carbon steel. The pipes and muffler are manufactured with fairly thin walls to help keep the exhaust system as light as possible.

The thin-walled construction and the use of special metals in the construction of the exhaust system can make weld repairs difficult to accomplish in the field. Most items are typically sent to specialty shops that have jigs to prevent warping and have the correct replacement metals and welding rods. 

The single pipes that connect the muffler to each individual cylinder are called risers. A set of pipes that connect each cylinder to the muffler, but which are joined together into one section before reaching the muffler, is referred to as a stack. The outlet from the muffler is the tailpipe. 

Each connection of one exhaust component to another can be accomplished by welding, by use of clamps, or by slip joints. 

Welding creates a strong, but rigid and immovable joint.

Clamped connections utilize clamp halves of various sizes. The clamp halves are connected with nuts and bolts that occasionally corrode over time and require replacement. The clamps can leak and require repositioning or retightening.

Slip joint connections have one pipe inserted a few inches into the adjoining pipe. The mating surfaces fit closely together, but allow for some movement. These connections make the exhaust system removal and installation easier and permit some movement to relieve operating stresses. 

It is important to lubricate all joints using a high-temperature MIL-PRF-907F qualified anti-sieze compound such as Locktite C5-A (up to 1,796 F) or Jet-Lube Nikal (up to 2,600 F).

 

 

Heat and corrosion

The exhaust system’s job is to remove all the hot exhaust gases from each cylinder head exhaust port and deposit them overboard. Exhaust components heat up quickly, but they also cool down rapidly at shutdown. Extreme temperatures and rapid temperature changes create an environment that produces metal fatigue and cracks. 

Connections such as slip joints, cylinder exhaust flange attachments and clamps can all deteriorate and begin to leak over time. Exhaust gas is very corrosive and can do severe damage to any metal component that is exposed to a leaking area. 

 

Muffler inspection

All heat deflectors and muffler shrouds should be removed when inspecting exhaust components. 

Bulges or wrinkles on the muffler are signs of overheating and metal fatigue. The sidewalls and lower sections of mufflers are prone to deteriorate and become thin. Areas where thinning is suspected can be probed with an awl to see if it punches through the material.

A bright light or an inspection camera can be used to check the internal structure of mufflers. Straight tailpipes provide a fairly large opening that allows access for a bright flashlight and inspection mirror. 

Exhaust systems with tailpipes that have bends or that are some distance away from the muffler itself can still be inspected internally by using an inspection camera with a small-diameter, long, flexible shaft.

Broken baffles that are loose in the muffler can be an immediate danger because they can partially cover the exhaust outlet. Jagged or distorted baffles can create hotspots that can cause premature metal fatigue.

 

 

Detecting exhaust leaks

Some exhaust leaks can be found by a visual inspection. Most exhaust leaks leave a gray or black sooty residue around the leaking area. Some leaks leave a yellow-tinted stain on the exhaust system itself. However, not all exhaust leaks leave a stained area behind. 

Some aircraft require substantial pressure test to pass. Always consult the aircraft maintenance manual to ensure that the mufflers are pressure-tested to the required point. Some only need 2-3 psi and some as high as 15 psi.

One of the most thorough ways to inspect an exhaust system for leaks is by use of a shop vacuum, some duct tape and a bottle of soapy water. 

To perform this inspection, the shop vacuum hose is inserted into a cold exhaust tailpipe and thoroughly sealed using tape. The vacuum switch and/or hose attachment on the vacuum should be set to “blow,” so that air blows out the hose rather than being pulled into the vacuum. 

Once the vacuum cleaner is turned on, the air blown in from the vacuum will pressurize the exhaust system enough to check for leaks. Slip joints, clamps and the welds around the muffler itself should all be sprayed with the soapy solution. Leakage will be immediately evident as the soap solution will begin to bubble. 

If it is possible to remove the components from the aircraft, a tank of water can be also be used to check for leaks.

The flange attachments on the cylinders are prone to leaking, especially on the cylinder flange attachments that have only two studs. Most Lycoming engines and a few Continental engines have the two-stud attachment. These flanges are elliptically-shaped, with a hole on each of the small sides. The flange on these connections is inserted over two studs on the cylinder port and drawn up tight with nuts. 

A gasket is used to seal the gap between the flange and the cylinder port. Over time, the flanges can become warped and get bent upward on the ends, leaving a gap in the center, which allows exhaust gas to leak past the gasket. 

Leaks in these areas can be detected by pressurizing the system as described above or by using a small feeler gauge to check for gaps in the gasket’s mating surfaces. 

If the flanges are warped, use of a spiral-wound gasket makes the problem worse. 

 

 

Heat damage and corrosion

As previously mentioned, leaks in exhaust systems should be immediately repaired, not only because of the danger they pose from possible carbon monoxide exposure or fire hazards, but also because the hot corrosive exhaust gas may cause rapid damage to anything that it blows on. This may include components near the exhaust, like the engine mount tubes shown in the picture (top, this page). 

  

 

Cylinder exhaust ports and exhaust flanges

The effects of exhaust heat and corrosion are especially pronounced on the cylinder flange attachments. (Refer to “Detecting exhaust leaks” on Page 30.
—Ed.) The cylinder port attachment for the exhaust flange is aluminum. The flat aluminum surface has two or more threaded inserts with steel studs installed. 

The aluminum degrades and pits quickly when leaking exhaust gaskets allow exhaust gas to blow out between the cylinder head and gasket. The gap and the exhaust leak will get larger with subsequent use if left unchecked. 

If the leak is not addressed, extreme pitting can occur to the point that resurfacing the pitted area on the cylinder is required. This is a laborious repair that involves removing the uneven metal to recreate a flat sealing surface. Surfaces that are severely eroded can’t be fixed and the cylinder must be replaced. 

There is no high-temperature silicone or sealant that will help to seal the gap. Proper sealing requires mating surfaces to be in contact with the exhaust gasket all the way around the gasket seal. Silicone is not effective as a sealing agent because it can’t withstand the extremely high exhaust gas temperatures.

In addition to causing erosion of the cylinder surface, exhaust leaks in the cylinder port area allow cold air to flow directly into the cylinder port through the gap. The cold air in the hot aluminum exhaust port causes the hot aluminum surface to crack. 

These cracks are not usually detectable during a cylinder compression test because they occur in the exhaust port area outside of the exhaust valve. When the exhaust valve is closed, this area is sealed from the combustion chamber. 

There is most likely a crack present in a cylinder exhaust port that has had a leaking exhaust gasket for several hours of operation, whether or not you can see the crack by visually inspecting the cylinder exhaust port. 

The exhaust port is typically covered in hardened exhaust deposits. The only way to remove the deposits is by use of a media blast material—chemical cleaners won’t cut through it—and this requires cylinder removal. 

An untreated gasket leak that causes an exhaust port to crack requires cylinder replacement. Replacing the cylinder is an expensive repair that is preventable by detecting and correcting exhaust leaks quickly.

The flanges on the exhaust risers themselves can be resurfaced by holding them flat on a belt sander and removing the high material until the flange has a perfectly flat surface again. This can only be done if the flange material on the riser is thick enough to allow some removal of the material without weakening the flange. If too much material is removed, the thin flange will become warped quickly and begin leaking again. It may also develop a crack. 

 

Hold-down studs

Leaking exhaust gaskets also corrode the hold-down studs to which the exhaust flange is attached. The threads can become damaged and cause stripped nuts that won’t tighten properly or hold torque. 

The studs also can vibrate and loosen in the threaded cylinder inserts. In this case, an oversize stud or new insert may be needed to hold the stud in place.

 

 

Exhaust gaskets

There are two main types of exhaust gaskets that are used on the cylinder flange attachments. 

The most popular and the longest-lasting are spiral-wound gaskets, also called “no-blo” gaskets. These gaskets are made of a thick carbon steel outer area surrounding an inner sealing area. The seal itself is made of layers of alternating stainless steel and asbestos. Spiral-wound gaskets are less prone to erosion and are more effective than other types of gaskets—provided the mating surfaces on the cylinder and exhaust flange are flat and not pitted. Some mechanics will reuse or reinstall spiral-wound gaskets when they are found to be in good condition.

The other gasket types (sometimes called “blo-proof” gaskets) are thinner and made of stainless steel or copper. These softer and more flexible gaskets are installed in pairs. They don’t last as long in service as spiral-wound gaskets, but they are more pliable and work better to help seal slightly uneven surfaces. The drawback with these coupled gaskets is that they will eventually begin leaking and so must be replaced periodically. In addition, if the exhaust system is removed for any reason, these gaskets may not be reused. 

 

Cracks and slip joint leaks

Leaks due to cracks or excessive leaks around slip joints can only be fixed by removing the parts and having them replaced or repaired. It’s best to send repair work to specialty shops that have the jigs to hold the parts in place as they are welded to prevent deformation. These specialized shops also have the correct repair material, which ensures a long-lasting weld repair. 

Some manufacturers recommend assembling slip joint connections with a small layer of ultra-high temperature anti-seize compound to help ensure a smooth disassembly later on down the road. 

 

Maintenance and operating tips

Here is a list of best practices to help preserve your airplane’s exhaust system.

• Pencils shouldn’t be used to mark exhaust components during maintenance because the graphite can weaken the metal as the exhaust heats up in use and cause a crack.

• Occasionally during an engine runup and magneto check, a pilot may accidentally turn the ignition switch all the way to “off” instead of selecting one magneto. If that happens, the engine starts to die. Most pilots will realize what has happened and will try to suddenly turn the switch back to “on.” This action can cause a severe and potentially damaging after-fire in the exhaust system. If the ignition switch is accidentally shut off during a magneto check, it’s best to let the engine shut down and then restart it. 

• As with all engine components, keeping the engine as cool as possible on climbout and as warm as possible on descent helps to minimize sudden temperature changes and extends the service life of exhaust components.

 

Conclusion

Detailed inspections completed at regular, close intervals can save aircraft owners money in the long run by catching problems before they do too much damage. Basic visual checks of the exhaust system should be a part of every preflight inspection; more detailed inspections should be performed regularly with the help of a qualified mechanic. 

Know your FAR/AIM and check with your mechanic before starting any work.

Jacqueline Shipe grew up in an aviation home. She soloed at age 16, has her CFII and ATP certificate and also obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. Shipe has also logged over 5,000 hours of flight instruction time. Send question or comments to .

Complying with McFarlane SB-9, Revision A – Universal Joints

Complying with McFarlane SB-9, Revision A – Universal Joints

Certain McFarlane universal joints for single-engine Cessna aircraft may fail and should be immediately inspected and/or replaced. The good news is that McFarlane has offered to help defray replacement costs for owners with affected parts.

On Dec. 11, 2017, McFarlane issued Service Bulletin SB-9, Revision A. This service bulletin addresses concerns with a replacement universal joint (Part No. MC0411257) that fits several models of single-engine Cessna airplanes, from the Cessna 120 all the way through the Cessna 210. 

The service bulletin applies to certain lot numbers of the universal joint that were sold in 2015 and 2016. The affected “Job Order” numbers in the bulletin include JO41505, JO41997, JO43600 and JO45454. 

 

Purpose of a universal joint

A universal joint is typically used to connect two shafts that are not directly in line with one another. The universal joint is hinged so that it flexes in two different planes that are 90 degrees apart. Another way to put it is that a universal joint flexes both up-and-down and from side to side. 

The universal joint that is the subject of the service bulletin is a bendable connection between the control wheel shaft and the Y-shaped control yoke assembly under the instrument panel. There are two universal joints installed in each aircraft, one on the pilot side and one on the co-pilot side. 

A failure and subsequent separation of either universal joint would render the control wheel on the side with the failed joint inoperative. Either the pilot or co-pilot—depending on which side failed—would no longer have control of the ailerons or elevator.

The McFarlane universal joint is designed with two yokes that provide the attach points for connecting the control wheel shaft and control yoke shaft; a center pin and two half-pins that the yokes pivot around; and a center block used as a base for mounting the pins. 

A rivet is used to secure the pins and yokes to the block. If the rivet were to break apart and slide out, the entire mechanism could come apart.

 

 

Service Bulletin SB-9, Revision A

It’s important to remember that service bulletins are not Airworthiness Directives. Even if a manufacturer refers to a service bulletin as mandatory, a private aircraft owner is under no FAA mandate to comply with it. As a result, some owners and mechanics may disregard service bulletins as being not all that serious. This service bulletin should be taken seriously, however, because the consequences of a failure of the universal joint are dire. 

Compliance with the service bulletin is straightforward. The initial inspection should be done immediately.  

The first step is to inspect the part and identify the “JO” (lot) number stamped on the universal joint itself to see if one of the affected joints is installed. The lot number and part number may or may not be visible by simply inspecting the joint without removing it. 

Some universal joints have protective sleeves made of rubber hose sections that are slid over the joint. Universals with sleeves installed may require removal so the sleeve can be slid back far enough to inspect the part and lot numbers. There are only two bolts and nuts that connect the universal to the system, so removal doesn’t usually take too long. 

If the joint part and lot number falls within the range listed, the universal joint should be removed and inspected.

The service bulletin states that if a joint passes the initial inspection, it must be re-inspected at 25-hour intervals. The joint may be kept in service for the next 100 flight hours or for one year after the first inspection, whichever comes first. After either of these limits is reached, the joint must be replaced.

Affected parts should be replaced as soon as possible, even if they pass inspection. Completion of the service bulletin by replacing the parts will provide peace of mind and negate the need for further inspections.  

 

 

Inspection of affected universal joints

Once removed, the inspection process for each universal is fairly simple. Each universal yoke should rotate freely in its range of motion. 

The yokes are designed to pivot around the center pin and half-pins. The pins should remain in a stationary position in the block and they should not move as the yokes are pivoted around them. If the yokes are bound or not freely pivoting, the universal joint must be replaced.

Service Bulletin SB-9 includes illustrations for marking the pins and yokes with a straight line using a permanent marker with the yokes held straight. Once the mark is made, bend each yoke; the line should appear broken as the yoke is pivoted around the pin. 

If the line remains unbroken as the yokes are bent, the pin is moving along with the yoke and the universal joint cannot remain in service.  

The center pin is easier to inspect than the two half pins that are installed under the rivet head and tail. Note that the illustration in Revision A of the service bulletin under the paragraph for the half pin inspection (See top image on Page 31) is actually an illustration of the center pin inspection. The concept is the same, however: the yokes should rotate around the pins while the pins themselves remain stationary. The rivet should be tight with no end-play or looseness. 

 

 

 

(Re)installation

Assuming that the joint passes inspection and it hasn’t exceeded the time limits specified in the service bulletin, it can be reinstalled. Any joint which fails the inspection should be replaced with a new MC0411257 joint. 

As noted above, one could argue that any joint from the affected lots (even one which passes inspection) should be replaced as soon as possible, as McFarlane is currently offering to assist in defraying the cost of replacement (see “Replacement parts” Page 35). 

Before installing new McFarlane universal joints that didn’t come directly from McFarlane Aviation (for example, parts from a maintenance shop’s spare stock), double-check to ensure they are not accidentally from one of the bad lots.

Some Cessna parts manuals specify that a protective sleeve be installed over the pivoting mechanism of the universal joint. The sleeve is made of a short section of reinforced hose and is listed under its own part number in the parts manual. This sleeve provides a small measure of support for the joint, in addition to protecting it from grit or debris that could cause it to bind. 

McFarlane specifies in SB-9, Revision A that it now requires the protective sleeve on all MC0411257 universal joint installations, whether or not the aircraft parts manual specifically lists it. 

The Cessna service manual recommends using general-purpose oil (MIL-L-7870) as a lubricant for the universal joints. McFarlane forbids the use of any type of penetrating oil like WD-40 on its universals. Most spray-can lubricants have penetrating qualities. General-purpose oil is heavier than penetrating oil and is usually applied with an old-fashioned hand-pump squirt can or with a brush. 

After installation, the control wheel should be moved from stop to stop in all directions to be sure that it moves freely with no binding. Replacement parts

Speaking as both a mechanic and a pilot, it’s commendable that McFarlane is currently offering both a parts and labor credit toward the replacement of affected universal joints. (As of early April, McFarlane has the parts in stock, ready to ship. —Ed.)

The service bulletin states “McFarlane Aviation, Inc. will issue a credit of $50.00 labor for the replacement of a universal joint from the affected lots (see Section IV). Part Credit and a surface freight allowance will be given upon return of the affected part(s).” 

This makes this a low-cost service bulletin for owners to comply with. 

 

 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Resources
UNIVERSAL JOINT MC0411257
McFarlane Aviation
 
FURTHER READING
McFarlane Service Bulletin SB-9, Revision A
 
CessnaFlyer.org/forum under “Magazine Extras”
Pre- and Post-overhaul: Engine Removal & Installation

Pre- and Post-overhaul: Engine Removal & Installation

Wise owners (and mechanics) know that a successful overhaul starts with careful engine removal. The overhaul process isn’t finished until after the engine has been reinstalled and the break-in period completed. A&P Jacqueline Shipe walks you through best practices to ensure start-to-finish success.

An engine overhaul is a daunting repair that usually takes several weeks to complete. In addition to the engine overhaul itself, there are several maintenance tasks that are associated with pulling the engine and reinstalling it after the overhaul. (For more about what comprises an engine overhaul, see “A Step-by-Step Guide to Overhauls” in the February 2018 issue. —Ed.)


Engine removal location and airframe storage

Once the decision to overhaul the engine has been made, the next step for an owner is to decide on the location for the engine removal. Some owners have their mechanic pull the engine and ship it to an overhaul facility. Other owners fly the airplane to the overhaul location and let the overhaul specialists remove, overhaul and reinstall the engine. 

The next task is to find out where the airplane will be stored while the engine is off the airframe. Hangar space is typically at a premium for both overhaul shops and general maintenance shops. 

Some shops place the airplane outside for the duration of time that the engine is off the airframe. The airframe is unbalanced and hard to secure on a tiedown once the engine has been removed. It is also much lighter than normal, leaving the aircraft more vulnerable to windy weather. 

Make sure to have a clear understanding with whomever is doing the engine removal and installation about where the airplane will be stored while the overhaul is taking place. 


Engine removal

Removing an engine from the airplane is typically not that time-consuming. The engine can be pulled easily enough in most cases in less than a day. 

Once the cowling and propeller are removed, the next step should be to take lots of pictures from all different angles of every section of the engine. This will help to determine the routing of hoses and control cables later on during the reinstallation process. 

The exact location of clamps is not usually specified by the maintenance manual and is left up to the mechanic. Knowing where the old clamps and supports were located helps ensure that everything fits properly during reinstallation.

Once all the engine components are disconnected from the airframe, the engine is stripped of everything that is not sent with the engine for the overhaul. The exhaust system, alternator, starter, vacuum pump and engine baffling typically don’t get sent in with the engine for overhaul. These components are either replaced or refurbished as needed by specialty shops. 

After all the necessary items are removed or disconnected from the engine, the engine itself is removed from the airframe. The tail of the airplane should be secured on a support that will hold it up once the heavy engine is removed. Most engines have permanent lifting eyes installed on one or more of the upper crankcase bolts. If an engine doesn’t have a lifting eye, one will have to be temporarily installed. 

A chain is most often used to attach an engine hoist to the lifting eye. Once the chain is secured, the engine hoist is raised until the chain has all the slack removed from it. Then, the bolts that secure the engine to the mount are removed from the vibration isolators and the engine can be lifted out of its mount. 

Once removed, the engine is either wheeled into the overhaul shop for disassembly or prepared for shipping if the overhaul is to take place elsewhere.

Engines that are shipped out by means of a freight company are generally bolted to a shipping pallet with a prefabricated mount. 

Owners that are having their engines sent out can save money by taking it themselves to the overhaul shop. The engine is often placed on a layer of used tires in the back of a truck and secured to four different tiedowns to keep it from shifting. 

In addition to saving money, the owner can have peace of mind knowing that he or she has overseen the engine shipment the entire time. Careless handling can damage expensive engine components and shipping companies do occasionally drop or damage items. 

If the overhaul facility is located a long distance from the aircraft location, shipping with a freight company may be the only option. In those cases, the shipment should be insured for the full replacement value of the engine. 

After the engine overhaul is underway, attention can be shifted to the repair or refurbishment of all the parts that are now easily accessible with the engine removed. 


Engine mount

Once the engine has been removed, the engine mount is easily accessible and can be thoroughly inspected for cracks and pitted areas. 

Even if the mount itself is in good shape, remove the mount from the airframe and inspect all the attachment areas on the airframe and mount for corrosion. 

Mounts that are free of corrosion and have good paint are often reused as-is. Mounts that are in need of repainting should be cleaned, lightly sanded and painted with a high-quality primer and then a coat of paint. 

In addition, any corroded areas on the airframe should be cleaned and treated or repaired as needed. 

Engine mounts that have pitted areas, excessive corrosion or cracks are usually sent to specialized welding shops like Acorn Welding or Kosola (now Aerospace Welding) for repair. These shops have special jigs and can cut out bad sections of tubing and weld in reinforced sections without distorting the shape of the mount. 

The firewall of the airframe is easily accessible with the engine and the mount removed. Now is an ideal time to clean and paint the firewall. Painting areas such as the firewall and the inside of the cowling with a bright color (usually white) helps to spot leaks easier. It also makes the airplane look better, and adds another layer of protection against corrosion.

Propellers

Controllable-pitch propellers and propeller governors are often overhauled at the same time as the engine. This ensures that the engine will be able to develop its maximum power within the proper limits without being held back by a sluggish or malfunctioning propeller or governor. 


Baffling

Metal engine baffles should be repaired as needed, and any worn baffle seals should be replaced to maximize engine cooling. 

Effective engine cooling is particularly important for overhauled engines because the new cylinder rings have to wear in and seat themselves against the cylinder walls during the first few engine runs. The extra friction will generate more heat than normal, especially in the cylinder heads. 

The air that the cylinders need for cooling should flow in through the front of the cowling, through the cylinder cooling fins, then down and out the bottom of the cowling. Any air leaks in the engine compartment that aren’t sealed off will allow cooling air to escape through a gap or hole instead of being ducted through the fins where it is most needed. 


Exhaust system

Exhaust system components are sent out for repair or are replaced if they are corroded, cracked or deformed in any way. Excessively thin or leaking pipes will only cause trouble later on. Leaking exhaust gases from warped exhaust flanges at the cylinder head connection will corrode and ruin the cylinder heads over time. 

Some overhaul facilities recommend replacing the exhaust system whenever the engine is overhauled. Turbochargers and wastegate assemblies should always be sent out for overhaul or replaced whenever the engine is overhauled. 


Hoses

All fluid-carrying hoses connected to the engine should be replaced at overhaul. Hoses become hardened and brittle after being heated and cooled during engine operation. A ruptured hose can cause a fire hazard or starve internal engine components of precious oil pressure. 

Also, tiny amounts of metal and debris can remain in old hoses even after they are rinsed and blown out and can contaminate the new engine. Many engine overhaul facilities will deem the engine warranty null and void if the fluid-carrying hoses aren’t replaced. 

It is also good idea to replace the SCAT hoses, but they aren’t critical like the fluid-carrying hoses are.


Oil coolers

Oil coolers should be replaced with new units or sent to an oil cooler specialty shop that can thoroughly clean the oil passageways. The oil passageways through the cooler have 180-degree turns in them that cause contaminants to precipitate out of the oil flow and build up in the turn areas.

It is impossible to get all the sludge, metal particles and dirt out of the old cooler by rinsing it in a parts cleaning vat. It’s not worth risking contaminating a freshly-overhauled engine with debris from the old engine in order to save a few dollars on the oil cooler. Clean oil coolers also have better oil flow through them and cool the oil more efficiently. 


Rubber vibration isolators

Most engines are mounted with the four attachments for securing the engine to the mount located on the rear of the engine. The rubber vibration isolators (often called “rubber engine mounts”) that are installed between the engine mounting pads and the engine mount should always be replaced whenever the engine is removed.

Vibration isolators lose elasticity over time and will begin to sag under the weight of the engine. Once the isolators start to age, they allow the front of the engine and the propeller to not only sag, but also to tilt down. 

The cowling is secured to the airframe and the propeller is connected directly to the engine, so as the engine mount isolators droop, the clearance between the bottom of the spinner bulkhead and the cowling becomes smaller while the gap between the top of the spinner bulkhead and the top cowling gets larger.

Isolators that are severely aged and distorted on these types of engine mounts can cause the engine to droop so much that the bottom of the spinner bulkhead actually starts rubbing on the lower engine cowling. 

In addition, rubber engine mounts are easily damaged and prematurely age if they are exposed to leaking oil or hot exhaust leaks. Constant oil leaks soften the rubber, causing it to swell and bulge. Exhaust leaks overheat the rubber, making it brittle and prone to cracking.

The isolators play a critical role in helping to secure the engine to the engine mount. They are typically not that expensive in comparison to other parts, and are easily accessible any time the engine is removed from the airframe—but difficult or impossible to replace without pulling the engine. 


Engine installation

The engine installation process takes longer to complete and is much more detailed than the engine removal process. Installing the engine mount on the airframe and then hanging the engine on the mount can be done quickly in most cases because there are usually only four bolts and nuts that secure the engine mount to the airframe, and an additional four bolts and nuts that secure the engine to the mount. 

Sometimes it is difficult to get the engine hoist adjusted just right so that the engine lines up correctly when attaching it to the mount. It can take a few attempts to get the bolts inserted through the mount and isolators. Components like the magnetos, fuel servo or carburetor may have to be removed to provide enough clearance to get the engine into the proper position on the mount. 

Engine mount bolts should always be torqued to the specified setting listed in the airframe maintenance manual and any specified torque sequence should be adhered to.

Once the engine has been hung, the baffling, accessories, hoses, oil coolers and all remaining parts can be installed. Clamping and securing hoses, wires and ignition leads is one of the most time-consuming tasks in this phase of the project. 

The exhaust system and propeller are usually two of the last items that are installed because once they are installed, they block access to other parts of the engine. 

Many overhaul shops run an engine on a test cell for an hour or so before sending the engine out. Some shops send the engine out with no run time on it at all. 

After reinstallation on the airplane, the engine should be started and run on the ground for the minimum time needed to ensure that there are no leaks; that the magnetos have the proper rpm drop when checked; and, if a controllable-pitch propeller is installed, that the propeller changes pitch as it should. 

Idle mixture and idle speeds should be checked and adjusted if necessary—but ground runs should be kept to a minimum, especially if the engine has not been on a test cell. 

After an overhaul, the rings are not seated. In order for the rings to seat properly, they must be blown out against the cylinder walls. The rings need high manifold pressures to force them to have metal-to-metal contact with the cylinder walls so they seat properly. 

Running an overhauled engine at too low of a throttle setting for any length of time (on the ground or in the air) increases the likelihood of glazing the cylinder walls. Glazing results from the oil oxidizing on the cylinder walls and creating a hardened surface that prevents the rings from ever seating properly. 

After the first flight, the cowling should be completely removed and the entire engine looked over for leaks and to make sure nothing has vibrated loose. Some shops will change the oil at this time if the test flight was the first run on the engine. 

The recommended break-in oil is generally used for the first 50 hours. After the 50-hour mark, there should be no metal in the oil filter when it is inspected. Metal found in the oil filter after this time may be indicative of an internal problem with the engine. 

Most overhauled engines perform well and provide many hours of trouble-free flight time and it is generally a relief for owners to have this major expense behind them.

 

 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

  

Mentioned in the article

ENGINE MOUNT WELDING
Acorn Welding Ltd. – CFA supporter

acornwelding.com

 

Aerospace Welding Minneapolis, Inc.

awi-ami.com

 

To find resources for other components and services for engine overhauls, please go to the Cessna Flyer Yellow Pages at cessnaflyer.org/cessna-yellow-pages.html or contact Kent Dellenbusch at  or 626-844-0125.

Engine Overhauls, Illustrated

Engine Overhauls, Illustrated

Most engines are “sent out” to specialty shops for overhaul. Peek behind the doors at Triad Aviation as author Jacqueline Shipe guides you through engine overhaul procedures. 

The single biggest repair expense most airplane owners will ever face is an engine overhaul. Overhaul costs increase every year along with parts prices. The engine overhaul process has become somewhat of a specialized procedure. Most mechanics won’t consider overhauling an engine themselves. The engine is typically removed and sent out for overhaul.  

 

When is an overhaul necessary?

The first step in the overhaul process is determining that an engine does in fact need an overhaul. Mere time since the last overhaul doesn’t always equate to needing to overhaul an engine. Part 135 operators must legally comply with engine manufacturers’ recommended times between overhauls. However, the only legal requirement for everyone else is engine condition. 

An engine that is run regularly (at least once a week) with cylinders that have good compressions with no exhaust valve leakage is a good candidate to keep running. Regular oil changes must consistently demonstrate that no excessive metal is being produced by the engine. Such an engine can safely and legally go beyond the manufacturer’s recommended time between overhauls (TBO). 

Cylinder issues can be resolved by replacing the affected cylinder, or by completing a “top” overhaul and replacing all the cylinders.

So, what might indicate it’s time for an overhaul? Excessive amounts of metal that have been determined to be coming from the bottom end parts (camshaft, lifter bodies, gears or crankshaft bearings) is one sign. If an engine has crankcase cracks that are outside allowable limits, it’s time. If an engine has problems producing its rated power even though cylinder compressions are good and fuel and ignition systems are within limits and working properly, an overhaul is likely needed in the near future. (For more, see “Is Your Engine Worn Out?” by Steve Ells in the October 2017 issue. —Ed.)

 

The overhaul process

An overhaul always includes a complete disassembly of the engine, thorough cleaning and inspection of parts, repair of parts as needed and disposal of defective parts. 

Major items such as the crankshaft, crankcase and connecting rods are subject to special inspections. 

Parts that are subjects of Airworthiness Directives or Service Bulletins are typically replaced or repaired in accordance with the steps outlined in the AD or bulletin. 

Parts are measured for excessive wear and proper clearances. The allowable dimensions and clearances are given in the manufacturer’s overhaul manual in two separate columns; one for manufacture (new) limits and one for service limits. The service limits are larger and allow for looser fits than manufacture limits. Some shops rebuild engines based on manufacture limits, while others use service limits. 

 

Crankshaft

The crankshaft is arguably the most important component in an aircraft engine. It absorbs the force generated by the reciprocating strokes of the pistons and rods and transforms it into rotational force for the propeller. The crankshaft is continuously subjected to loads and stresses from engine operation and the rotating propeller. Cracks or defects on a crankshaft can cause sudden engine failure or excessive, premature wear on the bearings. As a result, the crankshaft is probably the most inspected, measured and scrutinized part in the entire engine during the overhaul.

After engine disassembly, the crankshaft is cleaned and degreased in a chemical vat, dried and inspected. Most shops have a Magnaflux machine to inspect the crankshaft for cracks. 

The crankshaft is clamped between two copper-plated pads and an electric current is sent through the crankshaft to magnetize it. 

The crankshaft is then coated with a fluorescent solution containing magnetic particles. If there is a fracture in the crankshaft, the magnetic particles will align along the edges of the fracture. The fluorescent solution makes cracks easy to see under a black light. 

Once the magnetic particle inspection is complete, the crankshaft is cleaned again, and each journal is polished. Some shops have a machine that spins the crankshaft while a polishing rag is held stationary on one journal at a time with a special tool. Other shops use a machine with a circular cloth that is spun around each journal. The polishing process removes light scoring and surface corrosion as well as providing a clean journal surface so that good measurements can be obtained of each journal. 

Excessive scoring or pits caused by corrosion that cannot be removed by polishing the crankshaft can usually be removed by grinding off a specified amount of material. The manufacturer sets the sizes to which the crank can be reground, and it varies based on the engine model. Most Lycoming crankshafts can be ground to three-thousandths, six-thousandths or ten-thousandths of an inch undersize. Continental usually allows five-thousandths or ten-thousandths undersize. 

Once the crankshaft has been ground down to limits (referred to in the field as “ten under”), any further scoring or pitting defects in the journals will most likely result in the crankshaft being scrapped at the next overhaul. Reground crankshafts require oversize bearings to maintain proper clearances.

When all the machining and polishing processes are complete, the diameters of the main bearing journals and connecting rod bearing journals are measured with a micrometer at several points around the circumference of each journal. The smallest measured diameter is used to determine if each journal is within limits. 

The inside diameters of the connecting rod and crankcase main bearings are measured by installing the bearings and temporarily installing the bolts and nuts, securing the case halves and connecting rod halves together. A telescoping gauge is then used to measure the inside diameter of the bearings. Clearances are obtained by subtracting the journal diameter from the bearing internal diameter. Clearances must fall within the limits set by the manufacturer.

The crankshaft is also measured for straightness (or run-out) using a dial indicator. The crankshaft is placed in a holder that supports the crankshaft while still allowing it to rotate. A dial indicator reading is then usually taken on the rear main journal as well as the crankshaft flange. The readings must not exceed allowable limits. 

It is a fairly rare occurrence when a crankshaft is rejected. Aircraft crankshafts are constructed with high-quality metals at manufacture and, barring misuse or a prop strike, generally pass inspections through multiple overhauls. 

If the crankshaft needs to be replaced for any reason, it adds a significant amount to the cost of an overhaul. Some shops try to help owners by finding a serviceable used crankshaft, which is usually one-half to one-third the cost of a new crankshaft. 

 

Crankcase

The crankcase provides the housing to hold all the internal components (crankshaft, camshaft, rods) as well as providing a place to attach the cylinders, accessory case and oil sump. The crankcase is made of cast aluminum and must be strong enough to absorb all the opposing forces of the engine as it is in operation. 

Crankcases receive a thorough cleaning and inspection at overhaul. 

Some shops use abrasive media to clean the case and some use a chemical vat. Chemical-only cleaning processes are preferred because residue from blast material is difficult to remove from all the creases and recesses in the case. Any leftover media causes scratching and scoring once the engine is placed back in operation. 

Crankcases are inspected for cracks using a dye penetrant inspection. The case is saturated in fluorescent colored penetrant, then rinsed. The penetrant seeps into cracks making them easily seen once the case is sprayed with developer or examined under a black light.

Some cases are more prone to cracking than others. As an example, Lycoming “narrow deck” cases crack far more often than the thicker “wide deck” cases. Narrow deck cases utilize cylinders that have a thinner hold-down flange. The cylinder base nuts are Allen head (internal wrenching) types; while the wide deck cases have cylinders with thicker hold-down flange with standard six-sided nuts. Cracks can sometimes be welded and repaired depending on their location. 

Cases can have fretting damage or small areas of corrosion where the case halves are joined, especially near through-bolts. Cases with damage are generally sent to specialized machine shops such as DivCo or Crankcase Services to have the mating surfaces machined smooth. Some shops “line bore” the center bearing areas so that the crankshaft main bearings are perfectly straight and aligned with the each other. 

Regardless of whether the case is simply cleaned and inspected or sent out for further machine work, the mating surfaces of the case halves must be smooth and perfectly flat to ensure a proper seal once they are assembled. A silk thread is used to seal the case halves along with a special non-hardening compound designed to hold the thread in place as the case halves are assembled. Any irregularities in the mating surfaces will result in case leaks. 

Crankcases, like crankshafts, are expensive to replace and can add significantly to the cost of an overhaul if replacement is required. 

 

Connecting rods

Connecting rods are Magnafluxed, cleaned and dimensionally checked at overhaul. Connecting rod bearings along with the bolts and nuts that secure the rod halves are always replaced at overhaul. Connecting rod bushings are not always replaced, depending on the wear and condition on the bushings. 

The rods are checked with special dowel tools to be sure they aren’t bent or twisted. The connecting rod is turned sideways and held in a vertical plane. One dowel slides through the connecting rod bushing and the other through the crankshaft bearing. After they are inserted, the ends of the dowels are laid on perfectly-matched metal blocks. The four ends of each dowel pin should lay perfectly flat if the rod is not twisted at all. 

The dowels are left in place and a special gauge is attached to the end of the crankshaft bearing dowel. This gauge telescopes and it is extended until it touches the end of the shorter connecting rod bushing dowel.

After this measurement is made, the gauge is removed and placed on the opposite end of the crankshaft bearing dowel. If the rod is square and not bent, the gauge will line up and touch the short dowel on the opposite side without being extended or shortened.

 

Camshaft and lifters

The camshaft and lifter bodies are generally replaced or sent out to be reground to remove any light scoring marks or surface deformities. The camshaft lobes go through a carburizing process to harden them at manufacture. The depth of the carburized layer of metal is not very deep (about fifteen-thousandths of an inch) and it is possible for machine shops to accidentally grind below that layer. The camshaft lobe would wear down rapidly once placed in use if that happened. Additionally, the lobes are not only elliptically shaped, but they have a slight taper across the top of the lobe to ensure that the lifter body spins as it contacts the lobe. It takes very precise machine work when grinding the lobe to maintain its original shape and the taper across the top. Camshafts should only be sent to high-quality, experienced machine shops like Aircraft Specialties for machining work. 

Camshafts are not terribly expensive when purchased new (compared to major parts like crankshafts or cases). Typically, the cost of buying a new camshaft and all the lifters is only a few hundred dollars more than having the old ones reground. 

(For more on camshafts and lifters, see Jacqueline Shipe’s July 2017 article in Cessna Flyer. —Ed.)

Accessory case, oil sump, gears

The accessory case and oil sump are typically cleaned, inspected and reused. The Lycoming oil sumps that have intake pipes routed through the sump are reswedged around the intake pipe end to ensure there are no leaks down the road. This involves using a special tool which swells the pipe back out a little so that it forms a better seal when it is inserted into the sump opening.

The accessory case is inspected with dye penetrant and cleaned. The gears in the accessory case are cleaned, Magnafluxed and reused. 

 

Cylinders

Individual cylinder assemblies can be overhauled, but by the time the valves, guides and seats are replaced, the cost is almost equal to the cost of a new cylinder. Most overhaul facilities that I’m familiar with install new cylinders rather than overhauling the old ones. 

The cylinder must absorb the heat and pressure of combustion every time it completes a cycle while in operation. Metal fatigues over time and with a relatively low cost difference between new and overhauled cylinders, new cylinders are the best choice for long-lasting operation. They also typically come with their own warranties, so shops like them. 

It’s important to note that there is no logbook tracking for individual cylinder assemblies. Times in operation are kept of engines, but not of the individual engine parts. Therefore, it is impossible to really know how much operating time cylinders have on them when purchasing overhauled cylinders outright. The times that are on the existing installed cylinders on an engine can be difficult to trace unless they were new at the time of installation. 

 

Fuel system

The fuel injection system or carburetor is generally sent out for overhaul at a specialty shop or replaced with a new unit. Very few overhaul facilities overhaul the fuel system components in-house. Even Lycoming gets all the fuel injection system components and carburetors for both their new and rebuilt engines from Avstar Fuel Systems in Florida. 

 

Accessories and other items

All other accessories are typically sent to specialty shops for an overhaul or are replaced with new. Magnetos, ignition harnesses and vacuum pumps are generally replaced with new units. Alternators and starters are generally rebuilt. 

Oil coolers should always be sent out for specialized porting and cleaning to be sure all metal particles and sludge buildup is completely removed. The oil passages through the coolers make several 180-degree turns. Small metal particles and contaminants build up in the coolers around the curves and it is impossible to remove all the debris with just a simple flushing. Oftentimes, new oil coolers are fairly inexpensive, and it is easier and cheaper to simply replace them rather than overhaul them. 

All hoses should be replaced at overhaul. Hoses deteriorate with age and exposure to heat, and should be replaced periodically. New hose installations also help prevent contaminating the freshly overhauled engine with any sludge or debris remaining in the hose. 

It’s also a good idea to replace all the SCAT hoses. Most of the tubing (like the aluminum oil return lines) is cleaned, inspected and reused.

 

Choosing an overhaul facility

Engine overhauls are extremely expensive. When it’s time to overhaul an engine, choosing a high-quality facility to do the job is important. The best way to choose where to send an engine is usually by personal referral. Ask other owners what shop(s) they have used and what the long-term results have been. Owners or operators that have put three to five hundred hours on an engine usually know by that time whether the overhaul was a good one. Low cylinder compressions, oil leaks or other problems are signs that the overhaul may not have been the best. 

Most Part 91 owners only have to face an engine overhaul once. The process can be stressful to go through. Owners who do lots of research ahead of time, understand the process and ask lots of questions can help to avoid major problems down the road. 

 

 

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to

 

Resources

ENGINE OVERHAULS Airmark Overhaul airmarkoverhaul.com

Granite Air Center graniteair.com

 

Poplar Grove Airmotive poplargroveairmotive.com/engine-shop

RAM Aircraft ramaircraft.com

Triad Aviation hhtriad.com

 

CRANKCASE INSPECTION / REPAIR Aircraft Specialties Services aircraft-specialties.com

Crankcase Services, Inc. crankcaseservices.com

DivCo, Inc. divcoinc.com

 

FUEL SYSTEM OVERHAUL Avstar Fuel Systems, Inc. avstardirect.com

 

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Airplane maintenance for the DIYer: First Steps

Airplane maintenance for the DIYer: First Steps

 

A&P Jacqueline Shipe discusses why and how you can get started working on your own plane.

Owning an airplane is usually the result of years of hard work and planning. For many, it is a fun and rewarding experience—the fulfillment of a lifelong dream. 

Although airplane ownership is a big source of joy, it can also be an expensive and responsibility-filled endeavor. In fact, cost is the number-one concern that most pilots have when it comes to owning a plane. 

The initial purchase price of a plane is only one part of the equation. Insurance, fuel, storage, maintenance, avionics upgrades and any updates to the paint or interior can add up to be far more than the initial purchase price over a period of time.  

One way to lower the operating costs is to be actively involved in your plane’s maintenance. In addition to cleaning the plane, there is a surprisingly long list of maintenance actions that an owner may legally perform on his or her aircraft, provided it is not operated under FAR Parts 121, 129 or 135. 


The benefits of DIY maintenance

There are several benefits for owners who decide to do a lot of their own maintenance. Long-term, it does save on labor costs, although initially there are some expenses for tools and supplies. 

In addition to the cost savings, working on a plane gives a person the opportunity to get a better understanding of how different systems operate and how things on their aircraft are put together. This translates into a better understanding of the readings on the gauges in the cockpit and may allow the pilot to detect potential problems more quickly. 

Owner-performed maintenance also helps a pilot know how to operate the plane in a prudent manner that is easy on mechanical items. 

Owners also typically aren’t as pressed for time as mechanics working in a shop. This means that they can take the time to address cosmetic issues as well as maintenance issues. Little things like repainting removed items, fixing cracks in plastic or fiberglass trim pieces, or replacing rusted panel screws with stainless ones not only makes a plane look better, it adds to the resale value.


Preventive maintenance 

FAR 43 Appendix A, section (c) lists the maintenance tasks that an owner with a private pilot certificate is allowed to do and legally sign off. These all fall under the category of preventive maintenance, and the list is pretty extensive. 

A few of the items listed include tire changes, landing gear strut servicing, greasing wheel bearings, oil changes, fuel strainer cleaning, replacing or servicing the battery, and (with the exception of the control surfaces) even repainting a plane. 

Although these tasks are legal to perform, some of them are a little complicated, and the consequences if a mistake is made are high. Specialized tools and maintenance manuals are required for a number of the procedures. 

It is best for owners who decide to tackle some of these maintenance tasks themselves to pay a mechanic to show them the ropes for the first time. It is also a good idea for any owner to buy the latest revision of the parts and service manual for the specific make and year model of the plane he or she owns. 

Even folks that aren’t interested in maintaining their planes themselves can still benefit from a parts and service manual so they may look up part numbers, compare parts prices and have the information available in case the mechanic they work with doesn’t have it. (Mechanics have extensive libraries, but it is nice to supply them with complete paper copies that are easy to access.)


The necessary tools

In addition to the manuals, there are a few tools that are required for preventive maintenance. Most folks already have a general tool set for home use. The same items needed for tinkering on a car are needed for a plane: socket and wrench sets, screwdrivers, etc. 

A good ratcheting screwdriver that has separate bits works well for removing panels. The DeWalt brand Phillips drywall bits are great for removing stuck screws because the end is rounded so that more of the bit sinks into the screw head, making it easier to break the screw loose and less likely to round out the head. 

Screw guns really speed things up, but aren’t a necessity. A 7/8-inch socket made just for aviation spark plugs is nice to have also, and can be purchased from almost any aircraft parts distributor. 

Safety wire pliers and a can of .032 inch safety wire are handy to keep around. The oil filters and most of the bolts that require safety wire utilize this size. The pliers vary in price—from over $200 for high-quality ones, to around 20 bucks for a cheaper set. 

The better quality pliers are designed so that the teeth won’t gouge into the wire and weaken it as the pliers are clamped down. Safety wire can be twisted by hand; it’s just a little more difficult to do if you’re working in a tight place. 


Jacks and a tail weight

The biggest equipment investment that an owner who is really serious about maintaining his or her plane might want to consider investing in is a set of jacks. 

Planes with retractable landing gear need to be completely raised on jacks periodically for gear servicing or tire changes. Jacks can range in price from around $300 to a couple thousand dollars per jack, depending on the style and quality. 

The better ones have a long metal tube that slides over the hydraulic piston. This tube has holes drilled in slight increments along the length to allow a safety pin to be installed to prevent the jack from accidentally lowering if it loses hydraulic pressure. This type of jack is the safest, but it is a little more expensive than others. 

In addition to two jacks, a tail weight will be needed. These are fairly easy to make with some steel tubing and an old galvanized tub filled with concrete. The jack manufacturers also sell tail weight kits that are easy to assemble and fairly inexpensive; one just has to be sure the weight is heavy enough to counterbalance the heavy nose as the plane is lifted. 

Any time a plane is jacked, use caution to ensure it is being raised evenly on both sides. If the work is being done outside, make sure the wind is not forecast to get too high. Significant damage can occur to a plane if it falls off a jack.

Fixed-gear Cessna planes are the easiest planes to jack as they generally only require that one leg be raised at a time. A simple bottle or floor jack works well on the main gear legs. Some models have jack pads on the gear legs under the step; others require a removable jack pad. A jack pad is also pretty easy to make out of a heavy piece of angle iron. 


Logbook entries

After purchasing the proper tools and manuals, and with a little guidance, a person is well on his or her way to performing a variety of preventive maintenance items. 

Once a particular task has been completed, a logbook endorsement should be made stating the date, tachometer time, a description of what was done and the reference material that was used for completing the task. It is good to also include the part numbers for installed items. 

For example: “September 15, 2014; 2245 tach time; removed and replaced landing light bulb part number XXXX in accordance with Cessna Skylane service manual; operational check good.” The signature and the pilot certificate number of the person completing the work is what returns the airplane to service. 

Once a pilot gets started working on their plane, he or she may find it almost as rewarding an experience as flying it. The benefits for owners that learn to do a lot of their own maintenance can be well worth the initial investment in tools and materials.

Note: In future issues of Cessna Flyer, Jacqueline Shipe will be discussing specific preventive maintenance items step-by-step. 


Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s logged over 5,000 hours of flight instruction time. Send question or comments to .

Mind the Gap: Spark Plug Preventive Maintenance

Mind the Gap: Spark Plug Preventive Maintenance

In this installment of Cessna Flyer’s series on owner-performed preventive maintenance, A&P Jacqueline Shipe looks at the servicing and replacement of aviation spark plugs.

Aviation spark plugs need to operate while subjected to the wide temperature ranges that are possible in an aircraft engine. A spark plug with a 0.020-inch gap must be able to handle around 14,000 volts and fire reliably during its lifespan. 

Regular cleaning, gapping, and rotation of spark plugs helps ensure that the longest and most reliable service life for each plug is obtained. Regularly pulling and inspecting the plugs also helps diagnose cylinder health. 

Under Appendix A, paragraph (c) of FAR 43, the items “spark plug cleaning, gapping and replacement” are on the list of maintenance items an owner can perform on their own aircraft. 

Anatomy of a spark plug

Aviation spark plugs have a positive center electrode that is connected to the ignition lead terminal through a resistor. This center electrode assembly is housed in a ceramic insulator, which prevents the high voltage electrical current generated by the magneto from grounding out against the metal outer shell, which contains the negative electrode(s). 

These plugs are designed to withstand severe operating conditions and typically provide a long service life if they are properly maintained.

Removing the plugs

The first step in spark plug maintenance is removal of the plugs. Once the engine cowling is removed to the extent necessary so that access to all the plugs is achieved, the ignition leads can be disconnected from the spark plugs. 

The inner part of the lead needs to be held stationary as the outer nut is removed to prevent the lead from being twisted as the outer nut is turned. The leads should be gently pulled straight out and not cocked as they are removed from the plugs. 

A good deep-well six-point 7/8-inch socket is required to remove the plugs. Aviation spark plug manufacturers, including CFA supporter Tempest, makes and sells a specialized aviation spark plug socket that works well. Be sure the socket is properly seated on the plug before attempting to break it loose.   

It is important to keep track of which position each plug is removed from. This helps for diagnosing cylinder health and for plug rotation during the reinstallation. 

Homemade spark plug trays with marked receptacles for each plug are easy to make, or plugs can be laid out on a piece of marked cardboard. Tempest highly recommends using a spark plug tray to keep plugs from rolling of the workbench and to assist with proper plug rotation.

Avoid laying a plug on top of the cylinder, or any place where it could roll off and hit the floor. Dropped plugs often have cracked insulators or damaged resistors—and even if they pass a resistance check afterward, they could still have defects that can result in malfunctions and misfiring later on. Any plug that is dropped should be discarded. 

Some of the tools for performing preventive maintenance on aviation spark plugs include a spark plug tray, socket and torque wrench.
A view of the top section of an aviation spark plug looking down into the barrel at the ceramic insulator. The ignition lead terminal end is in the bottom of the barrel.
A Champion aviation spark plug socket is recommended for use with Champion spark plugs. Tempest also makes and sells a specialized aviation spark plug socket for its plugs.
This photo shows the proper wrench positions for ignition lead removal. The leads should be gently pulled straight out and not cocked as they are removed from the plugs.
Homemade spark plug trays with marked receptacles for each plug are easy to make. Plugs can also be laid out on a piece of marked cardboard. Tempest highly recommends using a spark plug tray to keep plugs from rolling of the workbench.
Inspecting the spark plugs

Plugs should be inspected after removal for excessive wear and general condition. 

Oil-soaked plugs

Any bottom plugs that are wet with oil aren’t a cause for concern, but if the top and the bottom plug in a cylinder are wet with oil, it can be a sign that there is either excessive piston ring wear, the ring gaps are lined up and/or the plug is malfunctioning. It wouldn’t hurt to take a compression check on the cylinder in question. 

Plugs that are misfiring will be oil-soaked simply because they aren’t firing enough to clean off any deposits; a top oil-soaked plug could simply be the result of the plug itself malfunctioning.    

Oil-fouled plugs should also be inspected for cracks and/or chips in the core nose insulator, according to John Herman at Tempest. Cracks or chips here may indicate a broken ring, which may result in cylinder damage from the broken piece of ring scoring the cylinder wall during piston cycles.  

Cylinders with insulator plug damage should be borescope inspected to be sure the cylinder has not been damaged or there is no evidence of foreign object damage or debris (FOD).

The amount of oil on this oil-soaked spark plug is such that it may not have been firing at all.

Taking note of buildup

Normally, any removed plug has a deposit residue of some sort on it and will be a little sooty just from the normal combustion process in the cylinder. 

Plugs that have virtually no deposits on them (i.e., too clean) or that have a slight reddish-brown tint on the insulator are indicative of a cylinder that is running too hot, or too lean, or both. 

If this is noticed only in one cylinder, the intake gasket and tube should be inspected for leaks. A partially clogged fuel injector on fuel-injected engines can also cause a cylinder to run lean. 

The most common deposits on spark plugs are lead and carbon. Lead buildup forms hardened balls that can eventually bridge the electrode gap and cause a plug to not fire. Carbon is jet black and sooty in appearance.

Excessive lead and carbon buildup on several plugs is a sign that an engine is being run too rich and not leaned properly. A good practice, endorsed by the folks at Tempest and others, is to lean on the ground any time the rpm is below 1,000. Always be sure to richen the mixture prior to takeoff. 

If the center electrodes are worn enough to allow passage through the hole in an erosion gauge, the plug is considered worn out and should be discarded.
On a fine wire spark plug, the electrodes are usually made of either platinum or iridium. Fine wire plugs don’t often (if ever) require re-gapping, and they last up to four times as long as massive electrode plugs. This plug was removed and cleaned but not reused and has now accumulated some rust on it.
Cleaning the plugs

Once the plugs are removed and organized as to which position they came from, the next step is to clean the plugs. 

Lead deposits can be very built up and hardened, making them difficult to remove. Safety glasses, a dust mask, and chemical resistant gloves should be worn to protect eyes, lungs, and hands during spark plug cleaning.

Vibration cleaning

Champion makes a machine that uses two cleaning prongs that vibrate at a high frequency to break loose the lead and pulverize it into fine particles that can be shaken out. Avoid breathing any of the dust generated from this process, as it contains lead particles.

These two-prong machines can be a little pricey, but there are handheld single-prong models that retail for a little over 20 dollars. (See Resources for a list of CFA supporters that sell the handheld spark plug vibrator cleaners. —Ed.) 

Abrasive blasting

In addition to getting the lead out of a plug, some shops clean the firing end of a spark plug in a sand or glass bead blast cabinet. 

Tempest does not recommend glass bead blasting on its plugs because some of the glass bead residue can become lodged between the center electrode and the ceramic insulator. As engine temperatures heat up, the glass beads melt into a conductive coating which can cause the plug to misfire. 

If a plug is to be blasted, Champion and Tempest both recommend using an abrasive grit that is made specifically for cleaning plugs. These companies advise lightly blasting only the tip of the plug; excessive blasting erodes the electrodes, causing premature wear. 

Some mechanics don’t recommend any kind of abrasive blasting to clean plugs due to the electrode erosion it can cause, especially on fine wire plugs. Tempest doesn’t recommend abrasive cleaning for its fine wire spark plugs for this very reason. 

Manual cleaning 

If plugs are oily, a little solvent (e.g., Varsol or other traditional mineral spirits) works well to clean the residue out of the firing end. Note: the plug should not be fully immersed in the fluid; it should only be used on the firing end. 

For stubborn lead deposits on the firing end, a good gun cleaning solvent, such as Hoppe’s #9 Bore Cleaning Solvent, is recommended by Tempest.   

A swab soaked in Methyl ethyl ketone (MEK, or butanone) works well to clean the insulator and ignition lead contact in the opposite end of the plug. Note: never use Tetra ethyl chloride on the terminal well area of the spark plug; rubbing alcohol will work just fine, according to Tempest.  

The threads on the firing end can be cleaned using a wire brush; just be sure not to clean the electrodes with the wire brush, as this can damage them. 

Gapping the plugs

Once the plugs are cleaned and dried, they are ready to gap. There are a few different styles of gapping tools, but they all essentially work the same. 

Re-gapping 

The plug is threaded into a receptacle on the tool, and a prong is pressed or screwed against the ground electrodes to move them closer to the center electrode. The recommended gap varies according to the plug and can be located on the spark plug manufacturer’s website. 

A wire-style feeler gauge is used to measure the gap between the center and outer electrodes. Care needs to be taken to not close the gap too much, as the electrodes can’t be spread back apart. 

Do not leave the feeler gauge between the electrodes when setting the gap. This can put a load on the insulator and cause it to crack.

Fine wire plugs typically don’t require re-gapping too often. Champion makes a specialized gapping tool for use on fine wire plugs if they do need to be reset. Tempest doesn’t currently have a similar tool, but is in the process of expanding its spark plug tool product line.  

Owner-performed maintenance includes spark plug cleaning, gapping and replacement. The photo at left shows how to use a spark plug gapping tool and wire-style feeler gauge. Take care not to close the gap on the plug too much, as the electrodes can’t be spread back apart.
The top side of a box of Champion spark plugs shows the manufacturer’s recommended plug rotation sequence.
A wire-style style spark plug gap gauge measures the gap between the center and outer electrodes. The recommended gap varies according to the plug.
 This photo shows two spark plugs and leads. After reinstallation, an engine runup and magneto check should always be performed to ensure that all of the plugs are firing properly.

Bench testing

Bench testing the plugs helps to detect and prevent reuse of a faulty plug. 

Both Tempest and Champion recommend the use of a bomb test to check a plug’s ability to fire under pressurized air. These types of testers are expensive and are usually found only in an equipped maintenance hangar, but it should cost only a few dollars to have the shop do the checks for you. 

A resistance test can be performed in addition to the bomb test, but it’s not a replacement for the bomb test. 

Tempest recommends using an electrical multimeter to check the resistance value between the ignition lead terminal in the upper part of the plug and the center electrode. The electrical resistance should not exceed 5,000 ohms on Tempest plugs. Any plugs with readings higher than 5,000 ohms should be discarded. 

Reinstalling the plugs

After the plugs are gapped, they are ready for reinstallation. 

Replacing gaskets

The copper gasket that seals the plug against the cylinder head hardens as engine temperatures heat and cools the gasket over a period of time. 

A hardened gasket does not seal as well as a soft gasket does, and can also keep the plug from properly seating against the cylinder head. Therefore, copper gaskets should be replaced before reinstalling the plugs. Spark plug manufacturers recommend that the gaskets are replaced each time the plug is removed and cleaned. 

Plugs that have thermocouple gaskets attached to CHT monitors do not require a copper washer in addition to the thermocouple washer.

Anti-seize lubricant 

Before the plug is threaded into the cylinder, a thin coat of a high-quality anti-seize material should be brushed on the threads. 

The first two threads closest to the electrodes should not be coated to prevent the conductive anti-seize compound from getting on the electrode and causing a misfire. 

Champion and Tempest make specialized anti-seize lubricants that they recommend for use on their plugs. 

A high-quality graphite- or copper-based anti-seize works well also. Aluminum-based anti-seize lubricants typically don’t work well because they don’t hold up under the severe heat. (Per Lycoming Service Instruction No. 1042AH "Use a copper-based anti-seize compound or engine oil on spark plug threads starting two full threads from the electrode, but DO NOT use a graphite-based compound.")

Rearranging the plugs

Aviation spark plugs should not be reinstalled in the same location they were removed from. 

Ignition leads are polarity-sensitive on all magnetos (other than some of the dual magneto models); this means that the north and south poles of the spinning magnet in the magneto generate a negatively-charged spark that is sent down one lead, alternately followed by a positively-charged spark sent down the next lead. 

Plug electrodes wear in predictable ways. The plugs connected to the positively-charged leads always fire from the positive center electrode to the negative electrodes, eroding the center electrode. The plugs on the negatively charged leads always fire from the negative electrodes back to the center electrode, eroding the outer electrodes. 

Keeping the plugs rotated so the positive and ground electrodes wear evenly will double spark plug life. They should also be rotated from top to bottom, as the bottom plugs usually incur more deposit material. A rotation that a lot of mechanics use is top-to-bottom, and next in firing order. 

Torque values

Proper torque values should be used when reinstalling the plugs. Lycoming recommends 30 to 35 foot-pounds (420 inch-pounds); Continental recommends 25 to 30 foot-pounds (300 to 360 inch-pounds). 

The ignition leads should be installed with care, and the leads should not be allowed to twist as the outer nut is tightened. 

Mag check and troubleshooting

An engine runup and magneto check should always be performed to ensure that all of the plugs are firing properly. A smooth runup and magneto check indicate a job well done.

A rough-running engine during the magneto check is most likely indicative of a little debris or excess anti-seize on the electrodes of one of the plugs causing it to misfire or not fire at all. Take note of which magneto the engine is running rough on. 

Once the engine is shut down and cooled off, check to see which plugs are fired by the magneto in question by visually following each ignition lead from the rough magneto all the way out to each plug. 

These plugs can then be removed, and any debris can be gotten out with a small pick. Any anti-seize lubricant that has gotten on the electrode can be cleaned off with a little degreaser.

 

Over the last seven months, I’ve given you some general tips and step-by-step ways you can work on your own aircraft according to what’s allowed in FAR 43, Appendix A, paragraph (c). 

This DIY series, along with guidance from a trusted mechanic, should give you a better understanding of preventive maintenance on your airplane—and might even save you a little money in the long run.

 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send questions or comments to .

 

Resources

Aviation spark plugs

Tempest Plus

 – CFA supporter

tempestplus.com

 

Champion Aerospace, LLC

championaerospace.com

 

Spark plug vibrator cleaners

Aircraft Spruce & Specialty Co. 

– CFA supporter

aircraftspruce.com

 

Chief Aircraft 

– CFA supporter

chiefaircraft.com

 

December 2016

 

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