Hi I just did a compression test on my 95 diesel pull truck, the truck has different pistons than stock but i dont know what kind, they are suposed to be lower compression. i had an average number of 460psi on each cylinder but #3 was only 400. I squirted oil in each cylinder and every cylinder jumped up to around 520. what are the stock pistons supposed to make? how do I know if mine is ok? and is #3 Bad enough to worry about. thanks!

 

More important than the average number is the the difference between cylinders.
Cummins says no more than 10% difference.
I've usually seen numbers around 400.

Never use oil when checking compression on a diesel engine.
Good chance the cylinder will fire and your gauge will explode.

 

im not that great with math but i would say i have a problem with cylinder #3, cause it was 60 psi lower than the other 5