Field Overhauls

Some Thoughts on Having Your Engine Overhauled – “In The Field”

A field overhaul can be just as good as a factory one, but be careful. Don tells you how he does them.

Field overhaul. Does the term give you a vision of your Mooney parked beneath a large tree, the engine swinging from a limb with a couple of guys in overhauls working on it? Although I’ve seen A few examples like this, most field overhauls today are done correctly and are a viable alternative to having a factory overhaul done to the engine in you Mooney. Since I’m in the business for field overhauls on O-360 and IO-360 engines, I’d like to discuss what to look for in properly done field overhaul.

The FAA’s definition of an engine overhaul is described in FAR 43.2 as follows:

“Using methods, techniques, and practices acceptable to the Administrator, it has been disassembled, cleaned, inspected, repaired as necessary, and reassembled.” So, according to the FAA, it’s legal to take your 2000 hr. engine, disassemble it, clean the parts, inspect the parts to minimum standards, repair parts to minimum standards and reassemble the engine. And that’s all. While this may be legal, chances are you won’t make another TBO.

Each year we remove eight to twelve engines from Mooney airframes for field overhaul in our shop. Although we are considered the over hauler, our involvement is limited. Most of the components are repaired or refurbished by specialists.

The following is a typical sequence of events for our overhauls.

The engine, prop and all accessories are removed. The engine is completely disassembled, and each part cleaned and visually inspected for obvious defects.

The crankshaft, camshaft, lifter bodies, sump, accessory housing, gears, shafts, connecting rods and rocker arms are sent to either Aircraft Engine & Accessories in Dallas or ECI in San Antonio. Both of these companies are aircraft engine machine shops, have been in business many years and do excellent work. Once at the machine shop, a dimensional check is made of each part. Applicable Service bulletins and Instructions are complied with. All steel parts are magnafluxed and dye checked. Connecting rods are checked for straightness and re bushed. The crankshaft is checked, inspected and if necessary, reground, polished and renitrided. A service bulletin calls for machining and modification of the crankshaft gear and dowel pin to be completed at this time.

Lycoming powered “J” & “M” model Mooneys and all Continental powered Mooney’s have counterweighted crankshafts. These counterweights are removed, inspected and reinstalled using new pins, clips and bushings. The camshaft and lifter bodies are reground and limits rechecked. If limits are not met, the cam or lifters are rejected and red tagged. These parts are then replaced with serviceable or new parts. Each serviceable part will have a yellow tag attached and all rejected parts will have a red tag attached.

The accessory housing and sump are dye checked for cracks, studs replaced and surfaces reconditioned. One area that is often overlooked on the sump is the seating area for the intake tubes. If worn, a bushing can be machined and installed to restore limits.

Intake tubes on older Lycoming engines used a small o-ring to seal at the sump area. The seating area on the tube will likely be worn and we replace these tubes with a later style that uses a larger o-ring.

Almost every Lycoming case I’ve seen will have some sign of cracking at overhaul. I send all cases directly to Divco in Tulsa or ECI in San Antonio. At these facilities, they inspect and dimensionally check each case for cracks, fretting and applicable AD’s service bulletins and instructions. Fig.6 shows the two case halves of a Lycoming engine. At the top of the case you may notice that the camshaft uses the case for a bearing surface. If this bearing surface is out of tolerance, the case half parting surfaces are resurfaced and the crank and cam are then re bored.

Early Lycoming engines use thru bolts that hold the crankcase halves together. Half of these studs would be anchored to one case half and the rest to the other side. These studs have a larger diameter in the middle and are designed to steady and align the case halves once assembled. These studs were prone to wear over time and as they did, the case would move. This movement caused damage to the case halves mating surfaces called “fretting”.

Most aircraft engine crankcase parting surfaces use 00 silk thread for a gasket. When fretting occurs, this gasket of thread breaks down and oil leaks start appearing at the parting surface of the case. At overhaul, these older cases should have the service bulletin complied with to install dowel pins that help eliminate this problem. Steel parts such as intake tubes, rocker box covers, generator/alternator brackets and other reusable hardware parts are sent to shops that do re plaiting. Accessories are often overlooked or ignored on many overhauls. I feel they are an integral part of a good overhaul.

I send my props and governors to Byam Propeller, Ft. Worth, Texas. Depending on the time on the prop, I may request a full overhaul or a reseal. If you request an overhaul, they are required by the prop manufacturer to replace certain parts, good or bad. If you request a reseal, these same parts are inspected, but not replaced unless necessary. The prop governor is flushed and run on their test stand. If repairs or adjustments are made, they are made to manufactures specs.

I send my carburetors and fuel injection systems to Aircraft Fuel Injection, Dallas, Texas. It’s sometimes quicker to call Aviall or Superior and exchange the systems, but I’m not all that keen on assembly line overhauled accessories. I know the people overhauling this equipment, I’ve been in their shops and if I have a problem, I know who to call.

Magneto’s, starters, alternators and generators go to Select Aircraft Services, Lancaster airport, Lancaster, Texas. For overhaul. Their shop, also, is top notch. They do excellent work on all these accessories, especially mags. The owner, a 201 driver, got into the magneto repair business after several failures of the infamous “dual Mag” on his Mooney.

The oil cooler is sent to United Accessories, Dallas, TX. for flushing and testing. They repair or exchange damaged coolers. Engine mounts are bead blasted, inspected, repaired, modified per AD and powder coated.

All fuel ,oil and governor hoses are replaced with new hoses from either the Aviall or Superior hose shops. I recently removed an engine from a 65 “E” model for it’s second overhaul. The oil cooler hoses looked as though they were molded in their unflexable shape. The date code on the hoses was Sept. 1964. Cylinders, several choices. It depends on what type of flying and how often. If a person flies less than 50 hours a year, I usually recommend chrome cylinders. Chrome cylinders won’t rust with the limited use like steel cylinders will, but they will use more oil than steel cylinders. Recently I’ve started using the new Cerminil cylinders from ECI. They have been out for a while, but this time I waited a while, I jumped on the bandwagon early when they came out with cermichrome cylinders several years ago, which did not do that well. The cerminil cylinders are getting good reports. These cylinders come with new exhaust and intake valves, seats and guides installed.

Steel cylinders are great if you fly 75 or more hours a year. Broken in properly, they tend to dry up and use minimum oil. New cylinders kits are available from Lycoming, Superior and ECI. Cylinder kits come with the cylinder, valves, piston, rings, gaskets and rocker shafts ready to install. Piston pins and plugs are purchased separately.

Once the parts are back, it’s our turn to reassemble the engine and accessories. Both Lycoming and Continental publish a service bulletin outlining mandatory parts to be replaced at overhaul. We comply with these service bulletins and use FAA/PMA parts from Superior Air Parts. We reassemble the engine according to the manufacturers overhaul manual and parts book. Since Lycoming’s overhaul manual has not been updated since 1974, we must also refer to the latest service bulletins and instructions for current updates.

Once the engine is assembled, the accessories are installed. Mags, prop governor, new fuel pump, Overhauled starter, generator/alternator, carb or fuel injection, air & oil filters, new plugs and harness. We include a remanufactured vacuum pump from Holly Aero Inc, Big Sandy, Texas. New pumps are available also. I’ve had good luck with these pumps, but I recommend changing pumps every 500 hrs if you fly serious IFR.

The completed engine is installed on the mount using new Lord mounts and then installed on the aircraft. All new fuel and oil hoses are used. Throttle, mixture and prop control cables are inspected for damage and replaced as necessary. Those not needing replacement are re-lubed. Battery and electrical wires are replaced as necessary as well as EGT, TIT and CHT probes.

Most engines are serviced with a straight mineral oil for break in. I use Aeroshell 80 or 100. The absence of the “W” means it’s mineral oil. I recommend using this oil for at least 25 hours. 50 hours if oil consumption has not stabilized. Once the engine and prop are installed it’s time for the initial start. We have pre oiled the engine with a pressure pot before hand. First start is usually less than a minute and a thorough check for leaks is conducted. Two more starts for longer duration and all controls and systems are checked. If every thing checks out, it’s time to fly. I normally fly above the pattern for approximately one hour and back down for another inspection. Next flight I try for two hours, maybe Dallas or Bryan and return. These flights are made at high power settings of approximately 25″ and 2500 rpm at cruise and full power in climbs with attention to oil pressure and temps. Back on the ground, one more check and we deliver to the customer’s home base.

Hopefully this information will be helpful in your choice of overhauls and what to expect. Regardless of who you use for your next overhaul, do yourself one favor. In the engine logbook have the mechanic document every part by part number that was replaced as well as the vendor. During the recent Lycoming oil pump AD, many oil pumps were changed because the mechanic simply wrote,” engine overhauled.” and not listing any part numbers. I list every part in the logbook and on the invoice.

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Don Maxwell
903-643-9902 or e-mail:

MAPA Log, Volume 24, Number 7 (July 2001)