Originally Posted by
NutLoose
An odd one here is in piston engines, the likes of Lycoming are the only ones that can zero time a Lycoming engine, so an overhauled engine from another company will come back with zero hours since overhaul but with the hours the engine has run to date, so you can have an engine with 4000 hours but zero run, took a bit of explaining that one once.
Not exactly so. An engine zero-timed at the factory is not quite the same as the old engine. Zero-timing is a result of pool-based overhaul where lots of engines are completely disassembled. Parts that are beyond repair are rejected, and serviceable/repairable ones go into a pool, so they have a pool of crankcases, a pool of crankshafts, etc., out of which they make engines (to new engine tolerances). Thus, the parts of a zero-timed engine may have been in use but do not necessarily correspond to one and the same engine. Thus, the engine "personality" is lost, and the only way out is to start counting the operating time from zero again. The engine gets a new number, too (which may be a completely new number or e.g. the original crankcase number plus a suffix - it's up to the manufacturer to define it).