Thread: compression

Not sure what the norm is but has it been hot or close to getting hot?? Also have you looked at that cylinder with a flashlight, with the piston down you should be able to see if there is any damage to it? If its been warm it may be a head gasket. As far as compression numbers I am willing to bet 90 is close with the other 5 being there.

I have read several different times that all cylinders should be w/in 10% of each other, outside the 10% and the low one usually has something wrong, but not a certainty.

Pa Pa,a guide line I have been told is all cylinders should be within 10lbs of each other.Also 115 is a good standard.I would NOT trust the 65 all 90 I think would be fine.See you in the fall

90 would be OK if both were that. And yes they should be within 10%. Has the motor sat for a long time without being ran? If so, soak the low cylinder with PB Blaster or any good penetrating oil. The best according to a test done by a machine shop (and cheapest) is a 1:1 mixture of ATF and Acetone. Let it soak for several days then run the motor for a while and then recheck the compression. What happens is when the motor sits for a long time with 1 cylinder at the top of its stroke the rings stick causing low compression. Soaking it with penetrating oil frees the rings and restores compression. No guarantees but I have seen this work.

low cvomp. is on the middle cylinder right bank .

Soak that one with penetrating oil. Its worth a try.