Try testing compression on cylinder #1 when the #5 piston has a big hole in it. All the cylinders must be tested, and a percentage variation between them needs to be calculated. Compression tests in general are a very primitive way of monitoring the condition of the cylinders. A leakdown test where air pressure is introduced into the cylinder with the crankshaft locked is a much better indicator of the mechanical condition of the piston rings, valves, head gasket, etc. Depending on the skill of the operator and the testing procedure he uses, a compression test is sort of a "measure with a micrometer, mark with chalk, and cut with an ax" process. It's better than nothing, but not much.
Jerry
Just to add to Jerry's statement. I see it as a generic comp test tells you if there is a problem. The leak down test tells you where the problem is. I knew nothing about the leak down tester until I borrowed one and watched some videos how to do it.
