I was scratching my head for a couple days as my combustion chamber cc measurement and math were not coming close to what my compression gauge was saying. Changed pistons, changed rings, changed cylinders, even checked the gauge against an air compressor, finally borrowed a tester..... still reading low. I was about to deck my head some more, then it hit me
^this kind is the wrong kind to use on a small displacement engine. The hose attached to the gauge has a schrader valve at the coupler but the detachable hose that screws into the head has no valves in it at all. Therefore the detachable hose has to be counted as combustion chamber volume. No big deal on a large engine but when you're dealing with a combustion chamber of around 4cc's it greatly affects what the gauge shows.
^So I dug out one of these, took off the rubber thing, gutted a sparkplug and soldered the sparkplug remnants to the compression tester. Schrader valve now sits where the electrode would go and combustion chamber volume is dead on. Gauge now shows 150% of what it was reading before.
So before you go decking your head and seizing your engine make one of these up to throw in your tool box. Takes about a half hour and only took a propane torch and some solder.