Understanding Piston Engine Overhauls


The purpose of this blog is to acquaint the owner with the various types of options available for re-powering his or her aircraft, the differences between them and the impact these options have on both the cost of operation and the value of the aircraft.

Other than fuel, the single most expensive component of aircraft ownership is the cost of operating and maintaining its engine(s). No other component on the aircraft represents a larger liability to the owner than the powerplant that powers it and no other single item will add or detract more from its value. The purpose of this blog is to acquaint the owner with the various types of options available for re-powering his or her aircraft, the differences between them and the impact these options have on both the cost of operation and the value of the aircraft. In order to effectively discuss this topic, one first needs to understand the different types of repowering options and the differences between them.

New Engine

A NEW engine is an engine that has been manufactured from all new parts and tested by an FAA approved manufacturer. The engine will have no operating history except for test cell time when received. No FAA approved manufacturer can approve another entity to manufacture or assemble a NEW ENGINE. When the owner elects to re-power an aircraft with a new engine, they pay the cost of the core charge in the purchase of the engine and no trade-in core is required. I discuss the concept of an engine core and associated core charge later on in this blog. A factory new engine will also come with a factory new engine warranty and that warranty will be honored at virtually any shop authorized to perform work on the engine by the manufacturer.

Factory Rebuilt Engine

A FACTORY REBUILT engine, often referred to as FACTORY REMANUFACTURED or FACTORY EXCHANGE engine, is an engine that has been OVERHAULED by the manufacturer or an entity approved by the manufacturer to NEW LIMITS using new and used parts. At the current time, neither Teledyne Continental nor Textron Lycoming approve any other entity to REBUILD engines for them. A factory rebuilt engine is a "zero time" (0 SFRM) engine meaning it comes to you with a new logbook indicating zero hours total time in service, even though the engine may have had used components installed that have many hours of previous operating history. For years Textron Lycoming used the term Remanufactured in their advertising and commercial media to describe their factory rebuilt engines. Around 2001, Lycoming discontinued the use of the term remanufactured and started using the term REBUILT to describe their factory rebuilt engines. So, prior to 2001 when Lycoming used the term "remanufactured" to describe an engine, that engine should be considered to be a REBUILT engine.

It is important for the customer to understand when you purchase a FACTORY REBUILT engine, you can either provide your run-out engine to the manufacturer for rebuild or exchange your current engine for an "off the shelf" engine. By exchanging your engine, it may be possible to substantially cut your aircraft's out of service time since the manufacturer often stocks rebuilt engines in inventory and has them ready for immediate shipment. In either case however the customer must provide an engine CORE for exchange, which is usually the run-out engine being exchanged. Additionally, the customer is required to guarantee the integrity of the crankcase and the crankshaft of the run-out engine, meaning that these components must meet the standards required by the manufacturer in order to be rebuilt. When a customer orders a factory rebuild engine, the manufacturer ships the engine to the customer and invoices the customer for the cost of the rebuilt engine PLUS a CORE CHARGE for the run-out engine. Recently, this charge has increased from $12,000 to $18,000. When your run-out engine is removed from your aircraft, it is returned to the factory in the same shipping container that was used to ship your rebuilt engine. Once the factory has inspected the run-out engine crankcase and crankshaft, a credit for the cost of the CORE CHARGE is issued to the customer. However, if there is a defect in either one of these components the customer will be charged for the component. Be aware that whether or not your run-out engine meets the manufacturer's return specification is at the sole discretion of the engine manufacturer.

Engine Shop Engines

Throughout the world, there are a myriad of reputable shops that specialize in overhauling aircraft engines. An OVERHAULED ENGINE is an engine that has been disassembled, cleaned, inspected, repaired as necessary and tested using FAA approved procedures. The engine may be OVERHAULED to new limits or service limits and still be considered a FAA approved overhaul. New limits are the FAA approved fits and tolerances that a new engine is manufactured to. This may be accomplished using standard or approved undersized and oversized tolerances. Service limits are the FAA approved allowable wear fits and tolerances that a new limit part may deteriorate to and still be a useable component. This may also be accomplished using standard and approved undersized and oversized tolerances.

In the case of an overhauled engine, the engine's previous operating history is maintained, along with its logbooks, and it is returned to you with a logbook entry indicating zero time since major overhaul (0 SMOH) and a total time since new that is the same as before the overhaul.

It is vitally important that the customer understand to what limit standards the engine is being overhauled. Some engine shops will use completely new cylinder assemblies (cylinder, piston, rings, valves, springs, lifters, etc.) provided by the manufacture or third party PMA (Parts Manufacturing Authorization) suppliers that provide new cylinders manufactured to the same FAA standards as the OEM. Other shops will remove existing cylinders, hone the barrels and reassemble the cylinders with new, used or a combination of both parts that meet new or serviceable limits.

Field Overhaul

The FIELD overhauled engine is an overhaul that is typically done by a local shop. In the case of a field overhaul, your mechanic will most often disassemble your engine and send the various engine components out to shops that specialize in the repair and/or rebuilding of these components. The crankcase, cam shaft and crankshaft will all be reworked and shipped back with FAA 8130 "yellow tags" showing that the work was performed by an FAA approved facility and providing an audit trail for the work done on the engine. Cylinders may be sent out as well but the cost of new cylinder assemblies has dropped so much over the years that it is often more practical in many cases to replace used cylinder assemblies with new.

Once your mechanic has received the various reworked components, he will reassemble them and reinstall the engine in your airplane.

Top Overhaul

TOP OVERHAUL is a term used by the general aviation industry when all the cylinders on the engine are overhauled or replaced with new. Time since top overhaul is abbreviated TSTOH or STOH. For example, a classified ad that reads "50 STOH" indicates the aircraft has flown 50 hours since the engine had a top overhaul. A common mistake a new pilot or owner makes is to assume a top overhaul is like a complete, or "major" engine overhaul. A major overhaul requires that the engine be completely disassembled, inspected, certain parts of the engine replaced, and other parts brought to within serviceable limits or better. During a top overhaul all cylinders are removed, inspected, restored to at least serviceable limits, reassembled with various new parts, and reinstalled at the same time. The work done on the cylinders may vary. A top overhaul has no FAA regulatory status as there are no regulations or time limitations to define the overhaul. Furthermore, while a top overhaul will extend the life of an engine, it is considered a maintenance expense, not a capital improvement, and will not add to the appraised value of the aircraft.

When an engine is OVERHAULED or REBUILT the new parts that are used during the repair process can come from a variety of sources. An OEM (Original Equipment Manufacturer) part is a new part that is manufactured by the original engine manufacturer to stringent FAA standards. An AFTERMARKET part is a new part that is manufactured by someone other than the original engine manufacturer that meets or exceeds the same stringent FAA guidelines as a new OEM part.

Any other terms used to describe the work performed during a engine overhaul are defined by the person or entity using them. They have no official meaning and often times are very misleading. Terms like "overhauled to factory specs or tolerances", "rebuilt equivalent", "overhauled to like new condition" and "remanufactured to factory fits and limits" and any other terminology that isn't defined above needs to be investigated as to what those terms actually mean. You will probably find that advertisements and log entries that use undefined terminology are not really delivering what you think you are getting. There are specific requirements by the FAA for the use of the terms OVERHAULED and REBUILT in an engine's maintenance records. If these requirements are not met it is illegal to use the terms. Any terms other than those listed have no meaning in the eyes of the FAA and should not be accepted by you in your engine log books.

Now that we understand all the terms, let's put it all in a nut shell. Only the manufacturer can currently produce a new or rebuilt engine. Both new and rebuilt engines are built to new limits. A new engine will have all new O.E.M. parts. A rebuilt engine can be produced using a combination of used and new O.E.M. parts. An overhauled engine can be done to new limits or to service limits or a combination of the two using used parts and new O.E.M or new aftermarket parts. An overhauled engine comes to you with it's previous operating history intact and zero hours since major overhaul. A new or rebuilt engine comes to you with no previous operating history and zero hours time in service, even though, in the case of a rebuilt engine, some of the parts used may have a previous operating history.

Understanding these terms and the regulations that apply to them, may make the decisions that you have to make, at TBO time, a little easier.

All the best and keep the blue side up...

©2011-2013 by AAM Consulting LLC