JDX8 Compression Questions

[DLDeere]

I'm working on my 73 JDX8. I have great compression on both sides, but over 20 pounds difference. Mag side is 170 and PTO side is 150. When I squirt some oil into the cylinders, the right side jumps to 240 and the left to 170. (Tough to pull with that much compression!)

We pulled both heads. The PTO jug, had a small groove. I was able to get rid of that with the hone. Mag side was OK. Didn't have new rings or gaskets, so had to put it all back together with the old head gaskets, but gaskets and rings all looked OK. Oil seals were old and bad, so we also replaced those while we were at it.

Compression results were still the same. By the way, someone's been into this before. It's been bored out to .060 over.

I didn't think about checking the ring end gap when I had them out. Probably should have done that.

So this much compression is good, but are there problems in running it with that much difference? I'd like to get this figured out while the engine is still on the bench. Thanks for any help!

Howard

[JDT]

Howard any chance that the side with higher compression has a 400 head on it?

Did you check that both sides were bored 60 over? Maybe one is only 20 over.

You should be able to run it as is for a few miles than recheck compression to see it they are closer.

[JDT]

Or maybe a planned head?

[DLDeere]

Both heads appear to be the same. .060 is stamped on the top of both pistons. Maybe after honing the jug, the rings need time to reseat. Don't know if it's a planed head or if I would be able to tell. I think it could also be ring wear on the lower compression side, but still, it's very good. Any problem with 20 lbs difference?

[JDT]

My old rule is 10% difference.
That would be 17 lbs so it is real close.
I would run it and see what it tests at after a few miles.

When it reseats you may end up with 160 even side to side. Or better yet 175 both sides.

[DLDeere]

OK. I'll give it a try. Thanks!