What does a compression test tell you?

Answer 1

A compression test can tell you many things such as if you're getting blow by. which means one of two things either all the compression ring gaps on the piston line up in sync or possibly you have a crack in your cylinder. Also if you have the distributor shaft apart from the engine a compression test will help indicate when the proper piston is at TDC (top dead center).

If you get low compression on one cylinder, put a teaspoon of engine oil down the bore. If the compression improves, then you have a worn bore or rings. If there is no difference, you have a burnt valve.

If two adjacent cylinders are low, it is very likely that you have a blown cylinder head gasket between those cylinders. This could also include a warped cylinder head and may need skimming.