750-four minimum ring gap?

First time doing 750 rings. Two questions;

I know that on my 750-four engine, the maximum end gap for the two upper piston rings is .7mm (according to my shop manual) but does anyone know what the minimum ring gap is? Since the rings expand a bit with heat, this measurement seems important to me, but oddly it is not covered in the manual!

Second, the rails for my oil rings hardly stick out of the grooves, like the compression rings do (before being compressed). Is that normal?

The number I remember is there should be something like .004" - .005" end gap on the rings per inch diameter of cylinder bore to allow for expansion.

Hummm, not sure how to convert that into millimeters, but it seems important to me. Too wide and end gap can cause compression loss and some oil burning, stuff we don't want. But too narrow a gap can actually cause bore damage! My compression rings average out at about .045mm, Seems small. And my oil ring rails have almost no clearance at all. That just doesn't seem right. Can't understand why these measurements arn't in the manual!

polkat wrote:

Hummm, not sure how to convert that into millimeters, but it seems important to me. Too wide and end gap can cause compression loss and some oil burning, stuff we don't want. But too narrow a gap can actually cause bore damage! My compression rings average out at about .045mm, Seems small. And my oil ring rails have almost no clearance at all. That just doesn't seem right. Can't understand why these measurements arn't in the manual!

For comparison, KHI's FSM for '77~79KZ1000 specifies standard ring end gap 0.30~0.50mm for top and 2nd rings with 0.80 service limit. Measured close to bottom of cylinder where wear is lower.

Good Luck!

Okay, I can work with that. I guess my main concern here is the oil rings. Installed on the piston, the oil rails are flush with the piston, not sticking out a bit like the compression rings do. When I push one of the rails down the bore, there eoesn't seem to be any measurable gap at all. Is that correct for oil ring rails?

polkat wrote:

Hummm, not sure how to convert that into millimeters

1mm is about .040", 0.1mm is about .004"

polkat wrote:

Hummm, not sure how to convert that into millimeters, but it seems important to me. Too wide and end gap can cause compression loss and some oil burning, stuff we don't want.

As posted above by Patton, the service limit for end gap is about 0.8mm or about .032" which is a spark plug gap...... that's a wide gap. It turns out most of the ring blow by is not at the end gap, it is "around" the ring in the side gaps between the ring and piston. The side gap is narrower but the linear distance is so much more you lose most blow by there.

Here is some good ring info.
comp ring Minimum end gaps, are set at .0035" per inch of bore.
www.hastingsmfg.com/ServiceTips/ring_gaps.htm

Thanks for the info guys. I understand compression rings better now.

But as I mentioned, my main concern now is the oil rings. I just went out and tried fitting the rails in the bores (like with compression rings-I put the oil rails in the bore. Better then half of them were loose enough to just drop through the bores! What? So I realize that the expanders keep the rails apart, but do they also somehow force the rails outward? When I remount the old oil ring assemblies on the piston, they are flush with the pistons, and there's little or no gap in the rails. I don't see how this can work? I had planned to use the old rings again as they are otherwise in good shape. So I'm wondering if new oil rings would be the same way?