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Mrboykin0 08-22-2012 12:01 AM

How to Properly Read a Compression Test Gauge?
Pulling an engine out of an salvage yard
(Big Block Chevy)

I understand how to do the test.
assuming the engine is warm/hot

But What are numbers i should look out for?

What is considered red flag and what is not?
What is are good numbers?

and if the cylinder(s) has low numbers does that mean the piston is bad? is the engine worth pulling?
can the compression be TOO HIGH?

Please help me?
- Hardcore Novice

454C10 08-22-2012 04:54 AM

The numbers should all be about the same.

Stock BBC's are typically low compression so don't expect high numbers. 125 psi is about it.

flat cam lobes and worn out bearings will be the biggest issue, and that will be hard to see at a salvage yard.

cdminter59 08-22-2012 05:54 AM

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.

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