Although gasoline is highly volatile, it is only when gasoline is mixed with oxygen and the mixture compressed that an explosion will result when the mixture is ignited. This is the fundamental concept behind every internal combustion engine. Compression takes place within the cylinders as the pistons move upward. To prevent air leaks, a number of rings surround each piston. At the top of the cylinder is the cylinder head, which is where the valves are. Like the piston rings, the valves must seal tightly to prevent air leaks. Checking for ring defects or leaky valves is of vital importance.
Testing for Damaged Piston Rings
Pull the spark plug wires off all the spark plugs and remove all of the spark plugs with a socket wrench and a spark plug socket.
Screw the threaded tip of a compression gauge into one of the spark plug openings within the cylinder head.