This is the best way to do a compression test to help you determine the state of the rings, reagrdless if your gauge is accurate or not.
Start the sled and let it run for about 30 seconds and then check the compression, what you are looking for at this time is if there is a 5lb or greater difference from side to side and if there is then it is an indicator that the rings need to be replaced. next start the sled and let it run until it is at operating temps and the t-stat is open and the coolant is flowing, shut it off and check the compression now that it is hot if the compression is 7lbs ( or more ) less than when it was cold then it is an idicator that you need rings. For example the cold compression was 135lbs and the hot compression is 128lbs or less then it probably need rings and generaly if it is 10lbs less than cold it usually will need pistons also.
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