Engine Overhaul Terminology and Standards
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TBO time draws near and you need to decide on a course of action. You call around, talk to your maintenance facility, and find that you are totally confused because you don't understand or know the definitions of many of the terms used by the people that you have been talking to.

New limits, Service limits, Remanufactured, Rebuilt, New, Used, Overhauled, Like new, OEM, Aftermarket, what does it all mean?

Lets look at and define the terms that are approved to be used by the FAA.
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.

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.
An OVERHAULED ENGINE is an engine which 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. The engine's previous operating history is maintained and it is returned to you with zero time since major OVERHAUL and a total time since new that is the same as before the OVERHAUL.

A REBUILT ENGINE is an engine that has been OVERHAULED using new and used parts to NEW LIMITS by the manufacturer or an entity approved by the manufacturer. At the current time neither Teledyne Continental nor Textron Lycoming approve any other entity to REBUILD engines for them. The engine's previous operating history is eradicated and it comes to you with 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

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 O.E.M. part is a new part that is manufactured by the original engine manufacturer to stringent F.A.A. 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 F.A.A. guidelines as a new O.E.M. 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 made 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.

TBO :- One important measure of an aircraft engine's overall economics is how often it has to be overhauled, the so-called time between overhaul, typically seen as TBO or TBOH. The TBO is a time 'recommended' by the manufacturer and depending upon how the aircraft is being operated.

A New Engine :- is an engine that has been manufactured from all new parts and tested by an FAA approved manufacturer.

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.

Overhauled Engine :- is an engine which has been disassembled, cleaned, inspected, repaired as necessary and tested using FAA approved procedures.

TSMO :- Time Since Major Overhaul Amount of time accrued since the Major Overhaul.

Total Time [ TT ] :- Total Time Accumulated on the component.

Tach Time :- Time in hours recorded on the Tachometer.

A Rebuilt Engine :- is an engine that has been overhauled using new and used parts to new limits by the manufacturer or an entity approved by the manufacturer.

Aftermarket Part :- is a new part that is manufactured by someone other than the original engine manufacturer.

Manufacture Minmum-Maximum :- Dimensions and clearances that are considered to be factory new.