Compression test for a newly reconditioned diesel engine?

A new engine that has not been ran to seat in the rings will have very low compression. Until the engine has been ran through a break in period, a compression test would be a waste of time. Since cranking compression depends on a lot of things, like, compression ratio, valve overlap & valve timing, I do not know of any rule of thumb for static compression pressure. The old standby for many years has been 15W40 oil, but with changes in the diesel engine, this is not always the oil of choice, so it is a very good idea to use the lubricants that are specified by the engine manufacturer. The last engines I can think of that required a straight grade 40 weight oil were the 71 series Detroit Diesel engines. If you feel that you need to do a compression test, first make sure that you have the proper test equipment, make sure that the batteries are up to the task & that the starter is up to the task, then do the test.

Hi henry,

As per the first reply the secret is to put 'a squirt' of whatever oil you have in the sump down the hole ie before fitting the injectors and let this form a seal around the top ring.. You will get a decent reading and a decent compression to get it started! This simple precaution gives protection to the rings and bores during those critical early strokes before the bores get properly lubricated. This is especially crucial if you have fitted some high performance scrapers as part of the rebuild.

One warning, do not let the re-build idle for long as this can also leave the upper bore short of lube oil.

Good luck with start up.

Massey.