I measured the ring gap this morning and found some interesting numbers. Measured it 1.5" from the top and measured it 4" from the top (3.53" stroke) and the taper changed the gaps about .0025". I didn't back calculate what the bore diameter difference was. BUT the big difference is between the old and new rings, or at least the one of each I checked. The old ring had a gap of .055" at the bottom of the cylinder and .057" at the top of the cylinder where as the new ring had .0185" at the bottom and .021" at the top. That is a huge difference. Doing the calcs for the 3.905" bore, at .004" per inch of bore for N/A applications, .0156" is the minimum gap recommended, and using .006" per inch for boosted applications I get .0234" for a minimum, so .021" will probably get me by if I put a turbo on this engine because the cast pistons will probably require keeping the boost down anyways. I pulled the recommended gap numbers from the Mahle website, whom the rings are made by.

The tighter ring gap ought to help out in the compression and blow-by departments.

Stuck the pistons in it and retouched up the paint on the intake and exhaust manifolds as well as the head. I cleaned up the oil pan and will re-paint it tomorrow as well as touch up the paint on the other stuff. Tomorrow I'll assemble the head and start the final assembly of the engine.

Still some ragged looking pistons, but hopefully bumping the compression up a half point or so, theoretically improving air flow in the head and tightening up the piston ring gaps net me some gains in horsepower and reduce overall oil consumption.


Pistons all in and oil pickup installed.
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Rebuilding an OHC Pontiac 250 with EFI and a Turbo