Understanding Engines Part 2 – Since Major Overhaul

Last week we looked at the term “since major overhaul” (SMOH) and discussed a few tricks when reviewing classifieds.   We also discussed engine hours and the time between overhauls (TBO).   We learned that a low time engine, one that has low hours SMOH does not necessarily mean that the engine is healthy.  Other factors definitely come into play when looking at the condition of an engine.

So Where Do We Go Next?

In this blog we want to discuss another misunderstood term that is often used in aircraft classifieds. “Since top overhaul” (STOH) is used quite often and once again is supposed to indicate that a particular engine is in good mechanical conditions.

An example of a classified ad may look something like this:

FOR SALE Cessna 172M, 5400TT, 1870SMOH, only 50 STOH…

Reading this ad it indicates that we have a Cessna 172M model for sale with 5400 hours on the airframe, 1870 since a major overhaul , and 50 hours since an engine top overhaul was completed. We know that a Cessna 172M has a Lycoming O-320-E2D, and has a TBO of 2000 hours. This aircraft has a higher time engine and depending on service history may require an overhaul shortly. The advertiser indicates that the engine has had a top overhaul only 50 hours ago suggesting that the engine is healthy and you should get many happy hours of flying out of it.

Well here is the problem.

So what is a top overhaul?

Engine manufactures outline what must be accomplish when completing an engine overhaul. A qualified individual or shop follows these instructions and once complete the engine is “zero” timed or has zero hours SMOH.  This type of work is expensive and depending on engine type can cost you well north of $20K.

As far as a top overhaul is concerned, no outlined guidelines are published. This means that there are no standards for a top overhaul. Therefore what one person may consider a top overhaul may not be the same thing for another. Take the above for example; the ad says that there are 50 hours since top overhaul. What does this mean? Have all cylinders been replaced? Mags inspected or replaced? New valves installed? Carb overhauled? Exactly what was done? I have seen these types of claims where the owner replaced one cylinder, checked the compression on the rest of the cylinders and called it a top overhaul!

When someone indicates that a top overhaul has been competed you have to find out exactly what work was accomplished. A top overhaul could be a good thing, as many parts and components may have been overhauled, repaired, or replaced, but it is NOT a substitution for a major overhaul.

We found the reason why higher time engines are given a “top” overhaul is due to cost. A “top” overhaul is a lot cheaper than a major overhaul and the owner is trying to squeeze every bit of life out of the engine.

Buyer beware.





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