A compression gauge or compression tester is a tool inserted into the spark plug hole on an engine while the flywheel is rotated. This will result in a pressure reading that can be used to judge the condition of a cylinder, piston, and rings. For a discussion on the ill-effects of certain compression testers being used on small displacement engines, see the Original thread.
One drawback to conventional compression testers is that they normally add to the combustion chamber volume. This is something that has been overlooked but in small displacement engines is very apparent. For a more accurate compression tester buy a press on compression tester, remove the rubber end, gut a spark plug and solder it to the compression tester.