>The compression test is done static. The engine is rotated to top dead
>center on
>the compression stroke. This ensures that both valves are closed. The
>propeller
>is held so the incoming air pressure doesn't kick the engine over and hurts
>someone. Then compressed air is pumped into the cylinder to a predetermined
>pressure. As I recall, it is 100 PSI. There are two gages that are read. The
>input pressure, and the pressure that is held by the cylinder. Cylinder
>pressure

Not to pick nits, but the standard reference pressure (aviation) is 80 PSI.

>If neither, but you can still hear air loss, it's
>rings that need replacing.

Piston ring leakage can be heard at the oil filler.

Of course you don't need a differential compression tester to isolate a
problem with rings or valves, just pump air into the suspect cylinder at
TDC and see (listen) where it exits. Some automotive compression testers
have a handy spark plug adapter that'll work.

================================
Angus Gordon                       '89 Carat
Delta, Ohio                            '86 Syncro