Piston ring gap

[Mark H.]

I am about ready to begin reassembly of my L75 motor. Using Keith Black 156 pistons on Eagle rods. Sealed Power E-251K rings. Bores are 40 over.

The KB installation instructions provide a gap factor of .0065 for a normally aspirated street engine. .0065 x 4.040 = 0.02626. Round up to .027?

Since no special instructions apply to the second ring or the oil ring, I went back to the Shop Manual and found gaps of 0.013 to 0.023 for the compression ring, which I understand to be applicable to the second ring. Is that correct?

For the oil ring, I get 0.015 to 0.055 gap.

i did all of this about 16 years ago but cannot find my notes and Google has not been as illuminating as I would like. I have two old threads from 2003 on this but they did not directly address the figures.

Thanks.

[Joe L.]

Re: Piston ring gap

I am about ready to begin reassembly of my L75 motor. Using Keith Black 156 pistons on Eagle rods. Sealed Power E-251K rings. Bores are 40 over.

The KB installation instructions provide a gap factor of .0065 for a normally aspirated street engine. .0065 x 4.040 = 0.02626. Round up to .027?

Since no special instructions apply to the second ring or the oil ring, I went back to the Shop Manual and found gaps of 0.013 to 0.023 for the compression ring, which I understand to be applicable to the second ring. Is that correct?

For the oil ring, I get 0.015 to 0.055 gap.

i did all of this about 16 years ago but cannot find my notes and Google has not been as illuminating as I would like. I have two old threads from 2003 on this but they did not directly address the figures.

Thanks.

Mark------


First of all, the top ring is alternately referred to as the top compression ring. The second ring is alternately referred to as the second compression ring.

For the top ring, you have the calculation correct for KB pistons. I would round to 0.026 but if you wish to round up to 0.027 that should be fine, too. Having the top ring at the KB specified end gap is very important when using KB hypereutectic pistons.

For the second ring, the specification for KB pistons is 0.004" per inch of bore diameter. So, in your case this would be 0.016".

Your 1st and 2nd rings should be pre-gapped at factory specs so you will likely need to increase them to the above-referenced.

The oil ring end gap should be set at factory specs. In your case this would be 0.065". SERVICE oil ring gap is high limit PRODUCTION (i.e. 0.055) PLUS 0.010. Your oil rings should be pre-gapped at this spec. but you should check it.

You also need to orient the ring gaps properly. For this, refer to the diagram in the factory service manual.

[Mark H.]

Re: Piston ring gap

Thanks Joe. Where did you find the KB reference for the second ring? I searched high and low for that and could not find it.

[Joe L.]

Re: Piston ring gap

Thanks Joe. Where did you find the KB reference for the second ring? I searched high and low for that and could not find it.

Mark------


It's in the installation instructions in the KB catalog or also found in the online KB Installation Instructions.

[Mark H.]

Re: Piston ring gap

And now I see it. Not sure how I missed it before. Thanks.

[Duke W.]

Re: Piston ring gap

Because of the high mounted top compression of KB hypereutectic pistons they run at higher temperature than conventional pistons and require GREATER THAN OE PISTON RING GAP. Failure to follow the KB instruction on the Web site (and instructions might be in the packaging) may result in ring butting that will break the top ring land.

This is unfortunately a common problem that is usually blamed on the pistons, BUT IT IS ABSOLUTELY, POSITIVELY DUE TO INSTALLATION ERROR - INSUFFICIENT TOP RING GAP!

Instructions are on the KB Web site. Google Keith Black pistons. I think the Web site is "united machining...." and a link to installation/ring gap instructions should be on the home page. For sure the top ring gap must be greater than the OE piston ring gap in the shop manual. I'm not sure about the second compression ring and oil control ring gaps, but it they are different that OE, the installation instructions will so state.

Duke