Continental A-65 compression test

My project plane came with a supposed 0 time SMOH Continental A-65 engine.  But it was 10 years in storage since the overhaul and, of course, that fact alone raised questions about the engines airworthiness.  So a compression test was the first order of business.

To test compression on a car engine, you screw a pressure gauge into the spark plug hole and then turn over the engine with the starter.  The compression of that cylinder is then read directly from the gauge.  We might call the car compression test a dynamic test since the engine is turning during the test.

In contrast, the compression test on an aircraft engine is a static test,  the engine is set at top center and left there during the test.  The test rig for an aircraft engine test has two gauges, the first one measures the input pressure...set at a standard 80psi.  The second gauge measures the pressure inside the cylinder.  Between the two gauges is an "orifice", a small hole.  If the cylinder being tested is perfect, there will be no air flow through the orifice and both gauges will read 80psi.  But if there is any leakage in the cylinder, the first guage will continue to read 80psi but the second guage with read some lower figure,  depending on how bad the leak.

The great thing about this kind of compression test is that you can actually hear the air leaking out of the cylinder.  An exhaust valve leak will be heard through the exhaust.  An intake valve leak through the intake system.  And leaking rings will make themselves known by a hissing in the crankcase.

Compression figures, then, are given in numbers like 78/80 (very good) or 52/80 (bad).  The FAA suggests maximum 25% leakage, so 60/80 would be the lowest compression figure you'd want to see. 

But the compression test is supposed to be done on a warm engine, just after running (hard to do on a project plane), and after the engine is well broken in (not possible on a zero time engine).  But, still, you can learn alot about an engine's condition in short order.

So, in November, we ordered a test rig from Aircraft Spruce and set up the test.  Sharon would hold the prop (they'll kick if you don't), and I would read the gauges.  It's easy to find top center, just hold a thumb over the spark plug while rotating the prop.  You can feel the pressure on compression stroke.  Then turn on the pressure on the test rig, read the gauges, listen for leaks...then go to the next cylinder.  The whole test didn't take 5 minutes.  The results:

Cylinder 1     0/80   leaking out the exhaust

Cylinder 2     65/80 leaking past the rings

Cylinder 3     30/80  leaking out the exhaust

Cylinder 4     75/80  leaking past the rings

Link to  Cylinders Overhaul,   Magnetos,   Engine Paint and Baffels

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