STOH vs SMOH

[swisspilot]

Can somebody explain me the difference?

Thanks

Alex

 

[NJA_Capt]

The simple answer....

STOH: (Since Top OverHaul) The case is not split, the overhaul involves the pistons rings and cylinders.
SMOH: (Since Major OverHaul) The case is split and all components are involved (incl crank, bearings and rods.)

 

[Roger, Roger]

The simple answer....

STOH: (Since Top OverHaul) The case is not split, the overhaul involves the pistons rings and cylinders.
SMOH: (Since Major OverHaul) The case is split and all components are involved (incl crank, bearings and rods.)

This.

Now, like anything in mx, the numbers don't tell the full story. I'd take an engine with a fresh top overhaul and 2000 hours since major that has flown every day, had regular oil and filter changes, had the mags inspected every 500 hours like they're supposed to be, etc etc over one with 500 hours since major that has been sitting for most of that time and has a dubious history on the "optional" maintenance items (oil and filter changes, mags, etc). In most cases, a properly assembled engine, that is flown regularly, and has the oil and filter (or at least the oil) changed every 50 hours, wear on the "bottom end" (crankshaft, camshaft, rods, accessory gearing) is actually quite slow and, barring a manufacturing defect in a part, will not cause a catastrophic engine failure even well past TBO.

 

[zulu99]

This.

Now, like anything in mx, the numbers don't tell the full story. I'd take an engine with a fresh top overhaul and 2000 hours since major that has flown every day, had regular oil and filter changes, had the mags inspected every 500 hours like they're supposed to be, etc etc over one with 500 hours since major that has been sitting for most of that time and has a dubious history on the "optional" maintenance items (oil and filter changes, mags, etc). In most cases, a properly assembled engine, that is flown regularly, and has the oil and filter (or at least the oil) changed every 50 hours, wear on the "bottom end" (crankshaft, camshaft, rods, accessory gearing) is actually quite slow and, barring a manufacturing defect in a part, will not cause a catastrophic engine failure even well past TBO.

What he said :yup: . It's most important that piston engines are run regularly. Internal corrosion of the engine happens much quicker than people realize. Those lycomings and continentals should really be run up at least once every week or two. If you know an engine will be sitting for more than a month or two it may be best to take appropriate measures to pickle. That is if you care about the longevity and health of said powerplant.

 

[USMCmech]

As a practical matter, unless metal starts showing up in the oil, there is no reason I would EVER split the case. If the bearings are going bad, then the report from the oil lab will show it. Otherwise, they are fine, don't try and fix them.


Contrary to popular belief, airplane engines do not "wear out". Engines are the most likely to fail in the first 100 hours after being built or overhauled. After that, if they are flown regulaurly, they'll run forever. I knew a pipeline patrol guy who had 8,000 hours on a engine. Flew 5 days a week, 4-6 hours per day, and had no plans to pull the engine anytime in the foreseeable future.

I once saw a diesel generator that was started in 1923, it had been running nonstop ever since when I saw it in 99.

 

[Maurus]

I once saw a diesel generator that was started in 1923, it had been running nonstop ever since when I saw it in 99.

Well that isn't a good example really. Keeping an engine running continuously instead of turning it on and off several times will actually help keep it from wearing out. It is a completely different situation.

One of the largest damaging effects to the engine during normal operations is starting the engine. Especially when people have too much throttle in and the engine starts at over 1500RPM. 2000RPM if you are a cirrus guy. A cold engine doesn't have the lubrication in the cylinders and a high RPM start is not good.

When I start an aircraft engine I personally aim for a 1000RPM or less start.

 

[USMCmech]

Well that isn't a good example really. Keeping an engine running continuously instead of turning it on and off several times will actually help keep it from wearing out. It is a completely different situation.

One of the largest damaging effects to the engine during normal operations is starting the engine.

Clearly that was an extreame example, but the basic priciple is true. Engines that run at (realitivly) constant power settings like airplane, ship, and generator powerplants last much longer than auto engines.