[Marine One]

There are a lot of ways to check "in cylinder performance". I have experimented with hot engines, cold engines, dry cylinders, wet cylinders, compressed air, compressed nitrogen, various grades of oil, etc. I have even gone as far as recording how much vaccum a cylinder pulls to understand how the rings seal to the tops of the ring grooves on the way down during the intake stroke. Much harder to get rings to seal while pulling vacuum.

Regardless of your method, you need to check frequently and you need to check consistently. Since you are trying to predict wear, you need to have a good benchmark of what is good before you can say what is bad. Get a baseline reading at rebuild and make it a point to check all cylinders again, the same way you established your baseline. Then measure and record everything the exact same way after you have burnt so many gallons of fuel. Example - at every 50 gallons of fuel burnt, measure, record and look for a trend.