REFLECTOR: Annual inspection/Compression Testing.

[davedent at comcast.net]

Related to compression testing.  I have done compression testing on aircraft engines from 1965, on all types of engines.  From R3350's to converted VW engines and it's all done the same.  Put an amount of air in and read the difference of what is lost across valves and rings and sometimes cracks in the cylinders.  But there is one thing I must remind others.  There are differences between manufacture procedures in the reading you see.  Also when the check is done.
  
It is always better to do the check hot because of clearances and oil sealing.  But in most cases and I would say 90% of the time it has had 30 minutes pass between shut down and testing.  So most test are done cold. 
 
The pressure readings are best done with a differential type gauge and with the larger continental engine an additional by-pass orifice is required.  So take note of this as well.  I like to see at least 70psi+ when testing, even cold. Even though the manufacture calls for a low as 58psi.  I listen for leaks pass the rings by listening at the oil filler port.  For intake valves at the air intake, and exhaust valves at the exhaust pipes.  But if above 70 I'm not to concerned. 

Now this is a story I want you all to remember.  
One day I was helping a fellow with his annual on his Piper 235 and doing a hot compression test. We had just run the engine with cowl off and were all set up to do the test.  I was working the gauges and he was holding the prop on top dead center.  I told him not to let go of the prop till I said let go and not until.  Well after about two cylinders, I said OK (instead of let go) and when he heard OK he assumed that I meant let go.  When he let go, the prop swung around and hit him with a glancing blow as he tried to duck and it took the top of his scalp clean off his head.
  
He went out across the hangar floor with blood flying everywhere.  I took a shop towel and tried to stop the bleeding, but had a hard time.  I put him my truck and rushed him to the hospital.  Fortunately at that time it was only a mile away or so.  He never lost conscious but it was a bad day for him and a day to remember. 
 
Remember you are putting 80psi against sometimes a surface of 5.2" and that's a lot of area to stop. So make sure you bleed the air off prior to letting go of that prop.  You could get hurt.  If you are using top dead center for the test and you should, and you have impulse mags, you can tell when you are on top by the mag snapping just when it goes though TDC. Otherwise you have to have your finger over that cylinder adaptor or plug hole to find it and if it's hot you won't want to do it.  Just some thoughts on the subject for the day.  
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