There is one thing during an annual that everyone dreads, the Compression Check, it is the one test that can make a cheap annual get very costly, but it is also the test that most mechanics have no idea what they are looking for or how to perform it correctly, most just want to see that number over 80, even though that number means little. Continental motors issued a service bulletin M84-15 on how to perform a compression check on any TCM engine. First thing is to calibrate your gauge with a master reference, mine calibrated out at 35, with the cylinder coming up on compression stroke, put 20 psi in the cylinder and continue to pull the piston to top dead center, then dial in the 80 psi on the left gauge, note the reading on your right gauge, is it above your reference point ? write it down, but you are not done yet, get a ear down to the oil tank, open the cap, what do you hear ? then do the same to the exhaust. The exhaust should be quiet, if not, time to dig a little, pull the cover and look at the exhaust valve, is it the same color as the intake ? or is there grey inside the spring and inside the cover ? that is the sign of a worn exhaust guide, and it is cheaper to fix now than waiting. If everything looks good, under the cover, get a bore scope and examine the exhaust valve itself, do you see any color spots, or is it the same all the around ? if it is the same, you are good to go. If you see isolated color spots, those could be signs of a hot spot and should be addressed, you can try lapping the valve in place or pull the cylinder and have a shop do it. One last thing, if you ever see a mechanic pumping a prop to get the numbers to come up, go elsewhere, I have never seen a engine do that in flight, and by starting with 20psi in the cylinder and pulling it to top dead, you are doing all you can to seat the rings to get the best possible seal.
If you do these steps, you will have a greater understanding of the condition of your engine cylinders.
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