Just thought I would reiterate some information I've mentioned before.
One of the most common ways a compression tester fails is due to a little known fact that the schrader valve that holds the compression in the tester so you can read it, is specific to compression testers and not a common schrader valve used in tire tubes or schrader valves for tubeless tires.
Typically what happens is someone goes to perform a compression test on an engine and the compression tester won't hold the pressure to be read. So they change out the schrader valve. Now it holds the pressure and the person doing the test thinks they fixed the compression tester and whatever results they measure must be correct. The readings now taken with the compression tester are low.
Schrader valves for compression testers have an extremely light spring compared to a schrader valve for tire schrader valves. Tire valves are meant to hold pressure and only allow air passage through the valve when the tire inflator mechanically presses on the core of the valve to open the valve to accept air. Have you ever had to stop at a gas station to put air into the tire and had to press the air chuck on the schrader valve so hard that you thought you might push the valve through the wheel rim before you heard the air going in to the tire? My point exactly. Unless the tire schrader valve is depressed, the likelyhood of getting air past the tire schrader valve is nil.
'The compression tester must allow the cylinder pressure to flow past the inner core of the schrader valve without mechanical assistance like the tire chuck and then hold that pressure and only an extremely light spring on the schrader valve will allow this to happen.
Another factor that contributes to low readings being read on compression testers is the fact that a lot of engines are now overbore engines (larger bore than stroke) by design with short connecting rods. This allows the manufacturers to go to 12:1 compression ratios (or higher) without reaching detonation and at the same time reducing the height of the engine saving weight and lowering the profiles of the vehicles. When taking the compression tests, the length of time the engine is actually at peak compression is greatly diminished compared to underbore (long stroke small bore)engines with long connecting rods. So the air being compressed and asked to pass through the schrader valve on the tester at peak pressure has less time to pass through on the overbore engines.
I firmly believe that compression testers are a good indicator of what's happening in an engine but the results are not the holy grail. I put all my faith in leak down testers. They tell the whole story. The results will tell you if you have a leaking intake valve, exhaust valve, head gasket, or piston rings and give you a quantitative value as a percentage.
Anyone who has made the switch to leak down testing, probably has a compression tester covered in dust somewhere in their tool box and the leak down tester is probably hanging proudly on the wall of the workshop. Once you make the switch, you won't go back. Leak down testing is idiot proof.
Would you go to a doctor and tell him you feel like crap and have the doctor put a stethoscope on your chest and then say " I'm not sure exactly what your problem is but if you allow me to do an exploratory operation, I can split you wide open and find the problem. I'll fix whatever I find and you'll feel much better" ? Doing and engine overhaul after finding low compression and not knowing what's causing the low compression is the way we did things back in the 1960's. I won't tear an engine down until I have a good picture of what I'm going after.
