Compression Ratios
A simple explanation that talks about diesels as well. When there is a loss of compression, the most likely reasons are: (1) A bad seal between the block and engine head which is the head gasket, (2) The piston rings are worn out, (3) The camshaft is worn so the valves are not opening and closing properly and (4) There is something physically wrong with one or more of the valves. These are not the only reasons, just the most common.