J25 Low Compression Help!!!

[avery-omc-parts]

I have a recently acquired J25 1926 with low compression.
What I have done so far;
Replaced Head and ball honed -Tried new rings-Checked piston specs. with caliper for out of round. Piston installed correctly.
Installed original rings, still no difference. Not as familiar with the 20-30’s motors as with the 50’s Need some advice!
I have another J with great compression, just cannot figure out what I may be missing on this motor. Am I correct that all that is in the compression is rings, piston and head on this style motor?

Thanks,
Steve

[jerry-ahrens]

The acceptable ring end gap is .005 to .012 thousandths according to my Johnson manual. You could check the ring end gap by placing a ring in the cylinder, then use the piston to push it in evenly. Then use a feeler gauge to check the end gap. Try several if not all of the rings. Be carefull, rings are easily broken. If it’s wider than spec, then your cylinder may be out of round, and or the rings are worn. You may be able to find a replacement cylinder along with piston rings to get it running again. There are quite a few of these motors around, so don’t give up on finding replacement parts.
Of course there is always the possibilty of an inaccurate gauge…. how much compression are you getting?

[avery-omc-parts]

I will check the rings and cylinders as you described. I don’t have the 18MM adapter for my compression gauge, I better order the adapter. Thanks, I will let you know.

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