[Cris]

Lugnutz:

Your leakdown tester has a small orifice between the two pressure gauges. In most automotive gauges, it is .040" diameter. When you hook your leakdown tester to your engine, it basically compares the leak of the engine against the .040" orifice in the gauge. Leave the output to the gauge open and you get 100% leak (the output gauge reads 0 psi), close it and you get 0% leak (the output gauge reads the same as the input gauge). As a sanity check, put a .040" orifice on the output of the gauge and you should read some kind of sensible leak. Apparently the theory is a little complicated to come up with an exact answer and the detailed flow characteristics of the hole also come into play. Really good leakdown testers come with a calibrated orifice that can be placed at the output of the leakdown tester.

Another alternative is to run a leakdown on a known good motor for a yardstick.

If it is the rings, you should be able to hear the crankcase getting pressurized. You could even seal the crankcase (the best you can) and place a small opening, maybe at the PCV outlet, and be able to feel or hear air coming out.

Another quick test is to run the leakdown, get a number, and then squirt oil into the cylinder, and run it again. A dramatically improved number suggests ring wear.

As you probably know, you don't have to undo the rockers to run the test, just get the cylinder to be tested at TDC on the compression stroke. Also beware, at 100 psi, the piston might actually move with the pressure.

Cris