Compression Tester Orifice

[cub builder]

Didn't I answer this in a previous thread? The answer is that it's arbitrary from ages ago (round engine days). It's not all that essential of a concern in doing the test.

Exactly right. The number isn't so important as whether the cylinder is leaking and what is leaking on it. Exh Valve, Intake Valve, rings, cracked head, scored cylinder? Large bore Continentals are expected to be down into the 50s or even 40s for compression as the ring gaps are huge and they tend to leak a lot cold. But it may be failing with a higher compression if it's a leaky valve. There is nothing magical about 60 PSI other than an arbitrary number that gets recorded in the logs so it is something people can read in hopes they are buying a healthy engine. Generally speaking, good compressions are indicative of a healthy engine. Low compressions may mean the engine is in for some major work, but not necessarily. They are only an indicator that the mechanic needs to look closer. I have seen a number of "failing" cylinders magically healed with a little TLC in the shop. No two gauge sets seem to read exactly the same, so there is apparently a lot of variance in the .040" orifices.

You'll also find compression differential testers used in racing, except that they often times use 100 PSI rather than 80. The differential compression or leak down test is more about showing potential problems so they can be repaired before they become real problema than finding a magic number to record in your log book.