There is a way to check the compression ratio w/o opening your engine. At my mechanic school, our teacher ask us to build something to check it. we took a big graduate tube at the chem lab. like 500ml. So we put a compresion guage attatch to the bottom, we fill the tube with engine oil (prevent rust which can occur if we had used water) at top dead center and turn it until bottom dead center. so

here's the theory

if we have 500ml a TDC and we have 42ml at BDC

500 divided by 42 = 12 times more room

so 12:1 of compression.

thanks.

 

http://www.sacoriver.net/~red/uccr.html

 

It's usually called 'CC'ing in the engine. The absolute most accurate way to do it is with the engine assembled as you have mentioned. Usually you coat the rings with a thick grease (like vaseline) so whatever measuring fluid you are using won't leak past them.

 

ok I doesn't know the name.