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Compression Check

2001-08-16 02:41:17

Compression Check

<HTML>Hey george

Can you tell me what's acceptable as far as compression checks?
During a compression check, is it acceptable for a cylinder to be leaking from an exhaust valve and/or is it acceptable to be leaking from the rings? I need your help.</HTML>

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2001-08-16 12:44:02

Re: Compression Check

<HTML>An interesting question that I can ask more about. Recently on my Lycoming O 360 A4M with 750 hours on chrome cylinders, numbers 2 and 4 check out 78 pounds hot or cold, anywhere within 20 degrees of TDC.

Cylinders 1 and 3 read 70 cold and 76 hot, but you have to go to TDC and get about 65 and then rock the propeller back and forth to seal the rings. No leakage through valves. I had cylinder 1 off recently for an oil leak and the engine shop said it had a lot of choke worn into it and not to expect more than1000 hours total from it.

It still makes full power on takeoff and cruises 2675 (digital) @ 7500’, same as always, with the same fuel flows.

Is this typical or unusual to check by rocking the prop and looking for the best? The trend is that the max is the same but it gets harder (more rocking) to get it.</HTML>

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2001-08-16 19:35:49

Re: Compression Check

<HTML>Compression checks are only a method to determine the relative health of the cylinder/valve/piston/ring relationship. It is commonly mistaken as a test of actual engine power capability. TCM has determined that compression readings as low as 40/80 can be acceptable as engines with compressions that low have still demonstrated the ability to develop rated horsepower for takeoff. (Their service bulletin regarding this issue specifies that the apparatus be properly calibrated.)
It's my opinion that when performing a compression check that compressions as low as 40 are permissible as long as the leakage is only past rings. If leakage is past an intake valve, the valve should be "staked" in an attempt to re-seat it, and the engine operated another 10 hours or so to recheck it. I do not believe it is wise to accept any leakage past an exhaust valve at all, excepting that if compression tests show a small leak, then staking the valve and operating only a couple of hours for the purpose of rechecking again. This has been shown to me to be sometimes caused by rust/corrosion on the seat of an inactive engine's exhaust valve, and the staking/and short operation has sometimes cured it. But a leaking exhaust valve is a serious problem and should not be accepted as normal.</HTML>