Why is top overhaul so common...

[Michael Burbidge]

I've been looking for a used engine and I often see ads that include a phrase something like, "1875 SMOH, 850 STOH". My question is, why is it so common for an engine to need a "top overhaul" at half the recommended TBO? Does this indicate abuse or improper care? or is it just to be expected from an aircraft engine?

Thanks,
Michael-

[LifeofReiley]

It can take a long time for most aircraft to reach engine TBO... sometimes 15-20 years. Most of the time you'll see this with Spam Cans, Experimentals just seem to fly more often. Low compression readings cause the top overhaul in most cases, usually signs of a plane sitting a lot and not being flown.

[Neal@F14]

Air-cooled aircraft cylinder heads really live a rough extreme hard life during normal operation, especially the exhaust valve components, mostly due to the extreme heating+cooling cycles and full-power continuous running for hours that such an engine must endure. Rarely when a cylinder needs "overhauled" does it have problems with the pistons or rings like in a car engine, but rather it's usually always valve issues instead. So, a top overhaul in an aircraft engine is really more of a glorified valve job than a real overhaul. Getting new piston rings in an overhauled cylinder is icing on the cake since new rings are dirt cheap and even if the piston is not re-useable and must be replaced, the pistons are relatively cheap as well.

The "bottom end" of an air-cooled Lyc is pretty much built like a juggernaut, and if there's no corrosion/pitting of the cam lobes/lifters, then it's not uncommon for the bottom end of an old O-320 or O-360 that has even been run well past the 2000 hour TBO to still look to be in fantastic good shape upon disassembly, however by then every gasket and seam on the whole engine will have been leaking oil like a sieve.

[mike newall]

I was Chief Pilot for a skydiving club for 10 years. We flew a Cessna 206, Piper Lance and Cherokee 6.

All of the motors had a top overhaul at about 1,000 hours, all 3 motors went to TBO and beyond - the Cherokee 6 O-540 had over 3200hrs on it when it finally went for overhaul - what a motor !

The Cessna succumbed to a crack in the case at about 2400 hrs and the Lance engine had done 3,000hrs when some muppet landed it gear up and ........

As previous posters have said - the top end gets a lot of work, if looked after, the rest will last for ages.

Let's look at smaller motors. The humble A-65 seldom gets beyond 500 hours before some tinkering is needed, so our 320 and 360's do very well.

As I preach all the time - regular oil changes, slow throttle movements and no operations above 1,000 rpm for the first 2 minutes after start all help to extend life.

[Kent Ashton]

Yeah, aside from the problem of just sitting and rusting, the valve stem lubrication system on Lycomings is pretty marginal (except for the H2AD) so the valves get loose and cylinders rust long before the bottom end gives trouble.

Compare the oil drainback tubes on a regular O-320/360 (3/8") to the same on an H2AD engine (1/2"). The H2AD engines have much better valve lubrication, it seems. -Kent