Compression test results

[ksuderman]

I had a pre-purchase inspection done on a Westerbeke 42B including a compression test. Everything I've read says that no cylinder should be more than 10% off from the others, and of course the engine has one cylinder that is exactly 10% lower (400 psi, 400psi, 380psi and 360psi). How concerned should I be? The mechanic wasn't overly concerned, and of course the broker is convinced it is not an issue, but I thought I would ask for opinions.

[tkeithlu]

I'm not overly concerned. Compression tests are open to a bunch of sources of error to begin with. That difference could be the amount of lube oil sealing the rings to the cylinder wall. The point is that the cylinders are not wildly out of whack with each other, and you have plenty of compression to fire. If she passes the smoke test (no white, no black, including on rapid acceleration) I'd accept it.

Don't take my reaction to the situation as gospel - others might disagree with me for good reasons or suggest other tests.