erikpmort

erikpmort

Well Known Member
what should be done for a motor's pre purchase inspection? I know they check compression, and i know you look at the logs for history, but what else?

I've got a 6 that i'm looking at- its been overhauled once, has a new carb and one replaced cyllander between the O/H and 1000 hours on it currently. Compressions are all mid 70s or better. What else do i need to check?
 
Well, if you have to ask, you probably need someone who knows engines and has the tools to evaluate it.
Check for any AD's, make sure they have been done and logged. A Boroscope is handy but if you don't know what your looking at, or where to look.....
I would not buy a used aircraft without a mechanic in tow for the FWF stuff, and I have "more than average" experience with engines, which is just enough to get me in trouble.
 
Checks

Think about the measurements from the previous overhaul.Will your expensive crank and case go another overhaul if ground again, or will you have to replace them. Has the engine had long times with no use, thus promoting corrosion? Any history of accident or sudden stoppage? Why was the engine previously removed from service? Was previous work done by name-brand shops?
 
Thanks, yes I would have an a&p there like the last time I bought a plane but I wasn't very impressed with the thoroughness last time so I'm looking for a list of stuff he should be interested in as well
 
Overhaul?

Hopefully there is a list of parts they replaced. If done correctly those called for in Lycomings mandatory replacement list were replaced and the owner has the receipts. Were the cylinders replaced with new or rebuilt? If rebuilt how many times/hours? We're the exhaust valves replaced, rods yellow tagged, rod bolts replaced? Accessories rebuilt? Case, crank and steel parts sent out and yellow tagged?
It would be nice to pull a jug and look at the cam, that's not really realistic to ask so settle for a oil filter inspection and/or oil analysis.
Did an A&P put his name and number in the log or was it done under a repairman's cert?
I think I would want to know all those things before making an offer.
Good luck!
Tim
 
Couple of thoughts -

See appendix D of FAR part 43 for a list of what's normally checked on a 100 hr / annual. (As you'll see, not too much can be done to see inside an engine.)

The thing I've noticed over the years with Lyccomings is that they can have great compression but very worn cam lobes / lifters.

The result is the cylinder seems fine, but essentially can't breath. So, it puts out much less power than spec.

If there has been oil analysis, check for trends in the metal - if not, what do the contents of the filter look like? (If the oil was very recently changed, there might not be much to see. Open up the filter and see, with a magnet, whether there is any amount of steel in the filter.)

One other idea would be to use an automotive type compression test. (You hook up the tester and spin the engine. (Disconnect plug leads and pull the top plugs.) The peak compression is recorded. Unlike the conventional leak-down test, the automotive test can tell you something about the condition of the intake as well.

So, the automotive test and a borescope exam would be on my short list.

Last ideas - you would probably want to make sure you had a current wobble test for the valve guides, the mags have been done within 500 hours, and the impulse coupling has been inspected.

Dan
 
engine health

When inspecting engines, one can easily inspect the "cheaper" parts, valves, cylinder bores, guides, etc. Even a cheap boroscope will allow a fairly difinitive view. Now comes the harder part, to inspect the expensive parts:
A weak part on Lycomings are the cam and lifters. when they start to pit, they will gall the cam, and start making metal. A good "tear open" of the filter sloshed in clean solvent, then place a magnet inside a "baggy" and see if ferrous metal clings to the baggy. Most early cam problems start with the fine metal in the filter. If the engine has been a "sitter", water vapor forming condensation (H2O) mixes with combustion sulphur dioxide (SO2) mixes to form H2SO4 or sulphuric acid that eats bearing material, hard to detect without splitting the case. My rule if thumb is if it has been ran regulary, the top endlooks ok and there is no ferrous metal in the filter, it is probably ok. If it has been a sitter with no preservation, it will need a premature tear down.

Regards,

Gary Brown [email protected]
 
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