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).
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).
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 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.
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.
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.
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.
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.
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 .
TCM Service Information Directive 05-01B, “Inspection Guidelines for CM Camshafts and Hydraulic Lifters”