The number everyone quotes
An engine is advertised at “600 SMOH,” and the buyer hears “nearly new.” SMOH — hours since major overhaul — is the most quoted and least understood number in a piston-aircraft sale. It does not tell you the engine's total time in service, it does not tell you how the overhaul was done, and it does not mean what the adjacent word “rebuilt” would mean. Those distinctions are defined in the regulations, and they change the value.
Overhauled and rebuilt are not synonyms
14 CFR 43.2 draws the line. A product may be called “overhauled” only after it has been “disassembled, cleaned, inspected, repaired as necessary, and reassembled” and tested to approved standards. It may be called “rebuilt” only if that work was done “to the same tolerances and limits as a new item, using either new parts or used parts that either conform to new part tolerances and limits or to approved oversized or undersized dimensions.” Every rebuilt engine has been overhauled; not every overhauled engine qualifies as rebuilt. The gap between those two words is measured in tolerances.
Only a rebuild resets the record
This is where SMOH misleads. A field overhaul, however good, does not erase the engine's history — its total time in service continues to run, and 91.417(a)(2)(iii) still requires the time since that overhaul to be tracked against the item's overhaul basis. A manufacturer rebuild is different: under 14 CFR 91.421, an engine “rebuilt by the manufacturer or an agency approved by the manufacturer” may be given “a new maintenance record, without previous operating history” — a genuine zero-time record. A field-overhauled engine described as “zero-timed” is a records contradiction, not a pedigree.
What SMOH still does not tell you
Even a legitimate overhaul number hides variables that matter. Overhaul to a manufacturer's “service limits” is not the same as overhaul to “new limits,” and both are lawfully called overhauled. A “top overhaul” that addressed only the cylinders is not a major overhaul that split the case, though both can shorten to “overhaul” in an advertisement. And for a Part 91 operator, the manufacturer's TBO is a recommendation, not a regulatory time limit — an engine can be operated past TBO on condition, and calendar time and disuse corrosion often matter more than the hour count. The number is a starting question, not an answer.
What cannot be concluded
Reading the overhaul entries, the total-time record, and the 8130-3 or shop teardown behind them flags terminology that does not match the paperwork and zero-time claims the records do not support. It does not establish remaining engine life, internal condition, or airworthiness — that requires physical assessment, including borescope and compression read together, and the responsible IA's determination against current manufacturer data.
What to request or verify next
- The overhaul work order and 8130-3 — confirm whether the engine was overhauled to new limits or service limits, and what was replaced versus reused.
- Confirmation of total time in service, separate from time since overhaul, and the basis for any “zero-time” claim (a manufacturer rebuild under 91.421, or nothing).
- Whether the event was a top or a major overhaul, and which accessories were included.
- Borescope and differential-compression results read together, plus any oil-analysis history, verified by the responsible IA.