How do you check them or change them?

 

you gotta know alot of engine part specs like piston height king pin height top shape rod lenght head cc thickness of the haedgasket.only way to lower or raise your compression of the engine is rebuild it. yes you can just change the heads or pistons or rods but if your already that deep in it id rebuild it.

 

I think this question is related to the 351 engine wear???? thread, in which a suggestion was made to "check the compression", and further explanation was requested. (A little patience, please - it does take most of us more than a minute and 5 seconds to type out a decent answer even if we see a question as soon as you hit the 'OK' button)

takeout... - Checking the compression [pressure] is a pretty simple test that reads the pressure created in each cylinder at cranking RPM, that you perform with a compression tester. It's related to compression ratio, but not directly (other things affect cranking compression).

Basically a pressure gauge that reads the cranking compression pressure is connected into into each plug hole (with the plugs out, obviously) while an assistant crank the engine over on the starter. Too little pressure indicates too much wear, and you can re-run the tests "wet" (using a small amount of oil) to help determine if it's more of a valve problem or a ring issue. You check all 8 cylinders, as a tired engine may not have the same problem in all of its cylinders.


Norm

 

As Norm said, the compression check cannot accurately tell you the compression ratio of the engine, but it does help diagnose the health of the engine. The main thing you're looking for is consistency. I've heard two views on this: one is the difference between the max cylinder and the minimum cylinder reading should be less than 10%, the other (more conservative) is the difference between max and min should be less than 5 psi.