The short answer
SMOH stands for Since Major Overhaul: the number of engine hours accumulated since the engine's last major overhaul. A listing that reads "1,200 SMOH" means the engine has run 1,200 hours since it was last completely overhauled.
What counts as a major overhaul
A major overhaul means the engine was disassembled, cleaned, inspected, repaired as necessary, and reassembled with parts within the manufacturer's new or overhaul limits. It resets the SMOH clock to zero. It is not the same as a factory-new or factory-rebuilt engine — a rebuilt engine gets a fresh logbook and a zero-time record, while an overhauled engine keeps its history.
SMOH is one of the first numbers buyers look at in an aircraft listing because it positions the engine against its TBO (time between overhauls). An engine at 1,900 SMOH with a 2,000-hour TBO is near the manufacturer's recommended overhaul interval; the same model at 400 SMOH has most of its recommended interval ahead of it.
Reading SMOH critically
Hours alone don't tell the story. Calendar time matters — manufacturers also recommend overhaul after a number of years (often 12) regardless of hours, because seals and internal surfaces deteriorate while an engine sits. A 500 SMOH engine overhauled 20 years ago and flown rarely can be a bigger question mark than a 1,500 SMOH engine flown weekly.
The overhaul's quality matters too: who performed it, to what limits (new limits vs. service limits), and what the logbook entry actually documents. A thorough maintenance history makes SMOH meaningful; a bare number in a listing is only a starting point.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.
Frequently asked questions
What does SMOH mean in aviation?
SMOH means Since Major Overhaul — the engine hours accumulated since the last major overhaul. "1,200 SMOH" means 1,200 hours since the engine was completely overhauled.
Is low SMOH always better?
Not by itself. Calendar age, how regularly the engine flew, who performed the overhaul, and the documented limits used all affect what a given SMOH number is worth. An engine that sits unused deteriorates even with few hours on it.
What is the difference between SMOH and SFRM?
SMOH counts hours since a major overhaul, which preserves the engine's total-time history. SFRM (Since Factory Remanufacture) counts hours since the factory rebuilt the engine and issued a new zero-time logbook. A factory-rebuilt engine is generally valued higher than a field overhaul at the same hours.