Those ‘70s Skyhawks

Those ‘70s Skyhawks

Over 16,000 Cessna 172K, L, M and N models left the factory in the 1970s. Most of these Skyhawks are still flying, and they’re gaining value, too.  

The Cessna 172 is the most successful General Aviation aircraft model of all time. It has weathered the storms of inflation, recession, crushing product liability lawsuits, and dwindling demand. 

The 172 was certified Nov. 4, 1955. A premium version called the Skyhawk was first sold alongside the standard 172 in 1961. The Skyhawk came with upgraded avionics and appearance packages. 


Four variants of the 172, the K, L, M and N models, and several modifications would take the 172 from 1970 through 1979. 

The 172K, L and M variants all came from the factory with the 150 hp, four-cylinder Lycoming O-320-E2D “Blue Streak” engine. The N model was powered by the 160 hp Lycoming O-320-H2AD.


Certified May 9, 1968, for the 1969 model year; 2,062 produced.

In 1969, the 172 had been upgraded with larger rear side window. Additionally, the rudder was fitted with a ground-adjustable trim tab at its base. In 1970, “drooping” conical-cambered fiberglass wingtips were added to the K model. 


Certified May 13, 1970; 1,535 produced.

In 1972, tubular-strut landing gear replaced the Wittman sprung-steel type gear. This change increased the footprint of the landing gear by 12 inches. The dorsal fin was lengthened to run the length of the fuselage to a point just behind the rear windows. 


Certified May 12, 1972; 6,825 produced.

With the M model, additional leading edge camber/droop was applied to the wings. This new wing was called the Camber-Lift wing and promised improved low-speed handling characteristics. Tinted skylights in the ceiling were offered as an option.

In 1974, the Skyhawk II trim package was added to the lineup. According to a Cessna marketing brochure from 1975, the Skyhawk II combined “…the businesslike blend of performance, economy and comfort [of the Skyhawk] with the nine most-wanted Skyhawk options.” 

The Skyhawk II trim package and Nav-Pac bundle grouped popular equipment options together to make ordering a new Cessna easier.


These two photos illustrate the differences between a 1975 172 sporting the Camber-Lift wing and extended dorsal fin and a 1963 model with straight wings and a smaller dorsal fin.

The brochure lists those nine options as: “Cessna Nav/Com with 360 channels for communications, and 160 for navigation with VOR indicator, Dual Controls, Emergency Locator Transmitter, Pitot Heating System, Alternate Static Source, Omni-Flash Beacon, True Airspeed Indicator, Navigation Light Detectors, Courtesy Lights.”

This cutaway drawing from a 1977 marketing brochure calls out the features of the Cessna 172.
Specifications sheet for the 1977 Cessna Skyhawk.
A fully-equipped Skyhawk panel, including the Nav-Pac option.

Certified May 17, 1976; 6,427 produced.

In 1976, Cessna stopped marketing the aircraft as the 172 and began exclusively using the Skyhawk designation. The “Skyhawk/100,” as Cessna called it, was introduced for the 1977 model year. 

The “100” moniker indicated that the aircraft was powered by a 160 hp Lycoming O-320-H2AD engine designed to run on 100LL. The previous O-320-E2D could run on 80/87 Avgas.

A Cessna dealer sales notebook lists increases in performance nearly across the board for the 1977 172 over the 1976 model.

In 1977, in-flight-adjustable rudder trim was available as an option, and pre-selectable flap control came standard. In 1978, a 28-volt electrical system was installed. In 1979, the flap extension speed was increased to 110 kias. 

This page from a Cessna dealer’s 1977 sales binder lists the performance improvements of that year’s new models.
Color choices and combinations for the 172 have tended to follow the trends of the times, as shown in this color chart for the 1974/75 model year.

The 172 continued to roll off the assembly line in the 1980s, until production was halted in 1986 due to unprofitability driven by onerous product liability lawsuits. The passage in 1994 of the General Aviation Revitalization Act (GARA) did indeed revitalize the General Aviation industry. Cessna began producing Skyhawks (and other aircraft) again in 1996. The Skyhawk continues to be produced today. 

In fact, as of this writing, the Cessna 172 may be more popular than ever. In an August 30, 2018 report from AOPA about the current hot market for used aircraft, the 172 is mentioned for its recent rapid appreciation (see link below). 

“Prices for 40-year-old airplanes do not typically jump 20 percent in the space of a few months, but that’s exactly what’s happened to Cessna Skyhawks produced between 1968 and 1976.” 

The report continues, “Rodney Martz, a senior aviation technical specialist in AOPA’s Pilot Information Center, said the recent jump in sale prices for Skyhawks produced in the 1960s and 1970s stands out as the largest percentage increase he has seen in many years, if ever.”

That, as we would say in the 1970s, is a really “far out” achievement. 

Jennifer Dellenbusch is president of the Cessna Flyer Association. Send questions or comments to .



Aircraft Owners and Pilots Association

Air Plains Services Announces New Engine STCs

Air Plains Services Announces New Engine STCs

Several Cessna Models Get New Powerplant Options


Air Plains Services announced three new engine STCs for Cessna piston singles during the AOPA Regional Fly-In at Santa Fe, New Mexico. Two of the STCs are for installation of Lycoming IO-540.

The addition of these three STCs—a 260-horsepower upgrade for Cessna 182S and T models, a 260-hp upgrade for Cessna 182RGs, and a 180-hp upgrade for Cessna 172R models—gives Air Plains one of the industry’s widest ranges of engine upgrades for Cessna’s most popular single-engine piston aircraft, the company said. As with other Air Plains engine upgrades, these modifications can be installed at the company’s Kansas facility or shipped as a complete kit anywhere in the world. 

“We’ve had a tremendous amount of interest and support for adding more options to our range of engine upgrades, and these three—the 260-horsepower upgrade for S- and T-model 182s in particular—are going to satisfy a lot of the global demand for better performance for these popular aircraft,” said Mike Kelley, Air Plains founder and owner. The three new STCs were originally developed by Alamo Aerospace and purchased by Air Plains, he added. “Customers for these new STCs can expect the same level of expertise and commitment to total customer satisfaction.”

“It just fits right in with what we’re doing,” added Air Plains’s Katie Church. “The heart and soul of our company is adding power to these Cessna singles.”

The engine upgrade for Cessna 182S and T models uses a new, 260-hp Lycoming IO-540 engine to provide a 13-percent increase in power over the original 230-hp engine. The upgrade can use the existing three-blade propeller to save cost, or use a new McCauley propeller. The existing propeller governor is converted to 2,700 rpm, and the upgrade includes a new tachometer. Performance enhancements include a 10-percent reduction in takeoff distance and a 20-percent increase in rate of climb. The new engine weighs essentially the same as the original, so there is little or no reduction in useful load, and no airframe modifications are required.

The retractable-gear Cessna 182 gains a second engine upgrade option from Air Plains, also using a 260-hp Lycoming IO-540. The kit includes a new McCauley three-blade propeller and spinner, a new electric fuel pump, governor, tachometer, and fuel flow/manifold pressure indicator. A new Slick dual magneto replaces the D2000 magneto, which is no longer supported by the manufacturer. The new fuel-injected engine provides an additional 25 hp for improved takeoff distance, climb, and cruise speed. Fuel injection improves fuel economy while eliminating the chance of carburetor ice; it also eliminates the horizontal carburetor, resulting in a more reliable, even fuel distribution.

The new 180-hp STC for Cessna 172R models is similar to Air Plains’s existing 180-hp upgrade and not only increases performance, but increases the useful load by 100 pounds. Because the upgrade uses the original engine, no airframe modifications are required. The kit includes a new propeller, tachometer, airspeed indicator, and fuel flow/EGT indicator.

“We are very excited about adding these new upgrades to our product range,” Kelley said. “This means more options and more opportunity to get the most out of your airplane for years to come.”

In the past year, Air Plains celebrated delivery of both its 2,500th 180-hp XP upgrade for Cessna 172s and its 500th 300-hp XP upgrade for Cessna 180s and 182s.

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 . 


Lycoming Mandatory Service Bulletin 518D


Lycoming Service Instruction No. 1565A

Record-Breaking Attendance Expected at the 14th Annual Gathering at Waupaca

Record-Breaking Attendance Expected at the 14th Annual Gathering at Waupaca

40+ Aviation companies step up with sponsorships, door prizes and seminars. 

Cessna Flyer and Piper Flyer Association members will be descending on Waupaca, WI for the 14th annual Gathering at Waupaca. Members from 20 states and Canada will spend the weekend relaxing, learning and mingling before boarding the air-conditioned motor coach to AirVenture on MondayTuesday and Wednesday with an optional night bus on Wednesday to accommodate viewing the night air show. 

Seminars for this year:

Electronic Ignitions by Michael Kobylik, Electroair; Brake Systems by Vern Rogers, Parker Hannifin; De-icing Principals by Ken Heath, UTC Aerospace; Getting the Most from your Engine by Neil George, Continental Motors; Fuel System Maintenance by Kurt Hartwig, Eagle Fuel Cells; Owner Performed Maintenance by Steve Ells, Contributing Editor. Meals, the bus ride and a full slate of seminars are all included in the event and the friendships formed last through the years. 

Save the date and plan to join us next year: Jul 20–21, 2019. Three exciting seminars are already booked for 2019 with more to come.

Platinum/Bus Sponsors

Tempest Plus |

 Electronics International |

 Univair |


T-shirt Sponsor

 Continental Motors |

Goodie Bag Sponsor

 Tanis |


Banquet Sponsor

SCS Interiors |


Bronze Sponsor

Air Capitol Dial |


Proud Supporters

 Aircraft Spruce & Specialty |

 AOPA Insurance |

City of Waupaca |

 Electroair |

 Flight Resource |

 Lycoming |

Mountain High Oxygen Systems |

Smooth Power |

Turbine Conversions |


Door Prizes:

Aerial Sim Training |

 Aero LEDs |

 Aerox |

Aircraft Spruce |

 AvBlend |

Bruce's Custom Covers |

 CiES |

 Concorde Battery |

 David Clark |

 Guardian Avionics |

 Great Lakes Aero Products |

 Icom |

JP Instruments |

Lycoming |

Lyons Studios |

McFarlane Aviation |

 Oasis Scientific |

 Parker |

Pilot Communications USA |

 Precise Flight |

Smooth Power |

 Sporty's |

Strut Wipe |

 Superbird |

 Superior Air Parts |

Turbine Conversions |

Van Bortel |

Vantage Plane Plastics |

Wag-Aero |

 Whelen |

Superior Air Parts Educational Forums at Aeroshell Oshkosh 2018 Forum Series

Superior Air Parts Educational Forums at Aeroshell Oshkosh 2018 Forum Series

Superior’s V.P. product support, Bill Ross (A&P/IA) will present two forums: Owner’s Guide to Engine Operations and Maintenance, and Engine Leaning Made Simple at the AeroShell Tent (#450) during Oshkosh/AirVenture 2018.

Coppell, TX (June 25, 2018)  — Scott Hayes, vice president, sales and marketing for Superior Air Parts, Inc., announced today that the company has accepted an invitation to be a presenter during the new AeroShell Forum series during Oshkosh AirVenture 2018.

AeroShell will host a series of forums on topics ranging from engine care to unleaded avgas. Follow them on Twitter @Shell_Aviation or visit the EAA Oshkosh AirVenture 2018 Schedule of Events at for updates.

Superior’s vice president of Product Support, Bill Ross, who has been an FAA A&P/IA for 33-years, will present two educational forums. Ross will present “Owner’s Guide to Engine Operations and Maintenance” on Tuesday, July 24, and “Engine Leaning Made Simple on Friday, July 27.” 

Both presentations will begin at 1:00 and be held at the AeroShell tent (#450), which is directly across from the AOPA exhibit.

“Superior Air Parts is extremely excited to be a part of the AeroShell Oshkosh Forum series,” Hayes said. “Helping pilots save money has been our philosophy for over 50 years and educating pilots on the many ways they can more efficiently operate their engines has proven to be a very effective way to do that.” 

Hayes also said that along with the two forums at the AeroShell tent, Ross will also be hosting the Superior Air Parts Oshkosh Forums. The free, 45-minute Forum sessions will be held daily at the Superior Air Parts tent (#257), which is just north of Hangar B. Forum times are 9:3011:00 and 12:30, Monday, July 23rd through Saturday, July 28th.  

In addition to all the other valuable information, all forum attendees will receive a free digital copy of Bill Ross’ popular 144-page book, “Engine Management 101.”

For more information and complete forum schedule, visit: www.

About Superior Air Parts, Inc.

Superior Air Parts, Inc., is a wholly owned subsidiary of the Superior Aviation Group. Founded in 1967, Superior Air Parts is the leading manufacturer of FAA approved aftermarket replacement parts for Lycoming and Continental aircraft engines. In addition, the company manufactures the FAA certified Vantage Engine and the XP-Series Engine family for experimental and sport aircraft builders. For more information, visit:


Dale Smith

Media Relations Representative 

Superior Air Parts, Inc.


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. 



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. 



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. 



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




Granite Air Center


Poplar Grove Airmotive

RAM Aircraft

Triad Aviation


CRANKCASE INSPECTION / REPAIR Aircraft Specialties Services

Crankcase Services, Inc.

DivCo, Inc.


FUEL SYSTEM OVERHAUL Avstar Fuel Systems, Inc.


Aircraft Accessories of Oklahoma

Curtis Superior Valve Co. Offers Aluminum Oil Quick Drain Valve for Lycoming Engine

Curtis Superior Valve Co. Offers Aluminum Oil Quick Drain Valve for Lycoming Engine

Curtis Superior Valve Co. Inc. has been a manufacturer of oil and fuel valves for the aviation industry for over 70 years. Curtis is proud to announce that they have received FAA PMA approval for the CCB-38000 low profile, aluminum oil quick drain valve. This drain valve fits most Lycoming engines.

When installed, the valve extends slightly more than the drain plug installed by Lycoming. The valve was designed specifically for retractable gear aircraft and twin engine aircraft with close fit cowling; however, it will fit any Lycoming engine where there might be interference from structure, exhaust, hoses, wiring or where weight is a concern.

The CCB-38000 valve comes with a separate activating tool (CCA-38001) that can be kept in the aircraft or mechanics tool box, and can be ordered separately if lost. Customers may purchase the CCB-38000 from Curtis’ worldwide distributor network. See for a distributor listing.