Here is Bill Cudney's compression test procedure...

This is the best way to do a compression test to help you determine the state of the rings, regardless if your gauge is accurate or not:

-Start the sled and let it run for about 30 seconds. After the 30 seconds is up, shut off sled and check the compression of all cylinders. If there is a 5lb, or greater, difference in compression from one cylinder to another, then it is an indicator that the rings need to be replaced.

-Next, start the sled and let it run until the motor is at operating temperature; ensure the thermostat is open and the coolant is flowing by either seeing a raise in water temperature at temp gauge or feeling for warm heat exchangers. Now shut off the sled and check the compression of all cylinders with the motor hot. If the difference in compression between this hot test and the previous cold test is 7lbs or more, it is an indicator that you need rings.

-For example; if the cold compression was 135lbs, and the hot compression is 128lbs or less, then it probably needs new rings.

-In addition, if the hot/cold compression difference is 10lbs or greater it is usually an indicator that new pistons are needed as well.

Thanks to DJM, Donna4010, and of course Bill Cudney.

FYI, edited by me for readability

Jerry C