When looking at aircraft for purchase online, a lot of acronyms are used. In particular, the SMOH and SFOH acronyms are thrown around a lot for used aircraft. What are the differences between these acronyms, and what are their purpose for a potential airplane buyer?
With many airplanes the longevity of the engine is a major factor in the price of the aircraft. Piston aircraft engines are generally good for 2000 hours of operation before they need to be overhauled, which is almost always painfully expensive. The more life left on the engine the higher the value of the airplane. SMOH and SFOH both have to do with the time since the engine was overhauled, both are in hours:
A factory overhaul means that the engine was sent back to the manufacturer for its overhaul, where a major overhaul could be done by anyone. In general factory overhauls are consistently good whereas non-factory overhauls vary more in quality. Factory overhauls are more expensive as a result.
If you see an airplane that has 300 hours since overhaul then there's plenty of life left, if you see one that has 1700 hours on the clock you know you are going to have to fork out a lot of cash much sooner, but you can get the airplane much cheaper.
Another acronym you may see is SPOH, that is Since Prop Overhaul. Variable pitch propellers also have mandatory servicing requirements, and although less expensive it's still not cheap, so a factor in the cost of the airplane.
The life of an aircraft engine is measured in hours of operation since some major service was done to it. When looking at ads for planes there are three regulatory terms that you'll see in engine descriptions:
You will sometimes see another term - "since top overhaul" (STOP or STOH), but this one is not actually defined in the regulations. Generally it means a shop removed and replaced all the cylinders (the "top end"), but did not open the crank case and thus may not have inspected the "bottom end" (crankshaft, camshaft, bearings, etc.)
There is no difference between a "factory" overhaul and someone else's "major" overhaul - at least according to the regulations: The standards for what constitutes an overhaul are the same. What is different is who you talk to if the engine breaks (for a factory overhaul you would go back to the factory, for an overhaul by an engine shop you would go to the shop, and for one by your local mechanic you would go talk to the mechanic).
If you're looking at a plane which has had an engine overhaul (the vast majority of the used piston GA fleet) you should pay attention to where it was done and the reputation of the overhauler, but given two engines, one overhauled at the Lycoming factory and the other at a reputable shop like Penn Yan Aero, they would generally be considered equivalent.