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 of Piper Flyer. —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 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 Piper 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.
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 .
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