compression ring question

CJim7

I havent pulled the piston from the dead cylinder yet to replace the rings, but since i havent dealt with this issue before, and im trying to do all my homework first before i start this...i have a couple questions. Not knowing the history on this motor, i dont know yet if it has been bored or not. Will i need to mic the bore or can i just go with piston diameter to determin what rings i will need? Secondly, i dont have the tools to install rings. Does anybody know from experience whether or not a place like Napa will install rings if i bring in the piston?

82Waggy

First, I'm not sure I understood the results of your compression test. Did it read 90? Have you pulled the head already? Any signs of a blown head gasket? If the pistons are overbore from standard, they may have a number on them like ".030". Measuring the piston should be accurate enough to find the ring size, but be advised that they are usually a few thousandths under bore size. Before you pull a piston I would take a close look at the valves.

tufcj

You can measure the piston or bore. 4.165 is stock, and go from there. Pistons are generally about .005" smaller than the bore. My 390 (same bore as 401) bored .030 to 4.195, the pistons measured about 4.190-4.191. A set of ring expander pliers and a ring groove cleaner are pretty cheap, probably less than $35 for a Craftsman set at your local Sears. Then a ball hone and a band ring compressor and you're set. Put a couple of pieces of old hose over the rod bolts so you don't nick the crank. They also can help guide the rod into place. Factory cast rings wore badly. The 360 in my CJ had about 83K on the clock when I got it. The factory rings were worn to just half the thickness of the replacements. Bob tufcj

CJim7

Thanks for the info.
To answer some questions:
I did a compression test and came up with virtually no compression on a dry test. The leakdown gave me 90psi which indicates rings.
No sign of a bad headgasket.
I have oil entering the combustion chamber.
I'll definately be checking the head over thoroughly when i remove it.

"Then a ball hone and a band ring compressor and you're set".
Bob, what's a ball hone and what would i use it for? (still doing my homework)

tufcj

A ball hone just has dozens of small balls on it instead of 3 flat stones like a conventional hone. It gives you a random pattern on the cylinder walls. You should hone the cylinder walls with 400-600 grit (depending on ring material) so the rings "seat in" properly. Bob tufcj