How to Properly Read a Compression Test Gauge?
Testing a salvage yard motor is not that hard. Have a squirt can of oil with you. Pull all the plugs. Remove anything else that will make it easier to turn the motor over with a socket and breaker bar. Check each cylinder writing down the reading of each. The readings should be within 10% of each other, If you have a low one try and squirt oil in that cylinder and take it again to see if it goes up. If not there is an internal problem. Ask if you can remove the oil pan and remove some main and rod caps to see how much bearing wear. Turning an engine over by hand I doubt you will have any high readings. Usually some yards have a way to set an engine up on a stand and run it for testing. This would be nice. You could see how much total vacuum the engine pulls. An used engine that pulled close to 15 LBS would be good.