I did a compression test.

The front engine has a compression value of 155 psi and the rear engine had a compression value of 120 psi. It was tested dry (without oil)

Then i tried a compression test with oil at the rear engine. Suddenly the compression went up to 145 psi.

So my conclusion is bad piston rings ore a bad cylinder.

My Question.

I would like to know how i can ben sure it's the piston ring ore the cylinder and what do i do next?

Where can i buy piston rings and do they need to be bigger then the originals ?

Did you check the valve clearances first