[gtobird]

With all of this fine advice, I checked the taper on the cylinders. As Cliff noted, engines with little ridge on the cylinders are often lower mileage engines and make for good re-ring jobs. This engine had by far the smallest ridges on the cylinders of the Pontiac engines I've done. The taper came out very nice. Two of the cylinders had .0015 taper and all the rest were less than that, so the wear was very even. Still need to check the pistons and crank. I expect the crank to come out fine as the bearings have nice even wear that's not excessive and the crank looks great just eyeballing it. Not sure how the pistons will come out. Crossing my fingers.

OJ

[400 Lemans]

Check the original pistons very carefully for cracks in the ring lands. Almost sprung for a re-ring job cause the bores were still decent but after another careful inspection i found three cracks on no. 7 piston. So since i had to buy new pistons anyway, i went ahead with .030 overbore and forged pistons. Some of the cracks are really hard to see. I used a magnifying glass to spot. From now on i run octane booster and 93 gas.

[Jay S]

I don’t have a lot to add too any of the comments. .005” bore taper is kind of my max that I will consider a rering. More than .005” across all holes or higher it needs bored. With .003” to .005” hold the RPMs to 5000, 5500 max. When he have tried too turn engines with that much bore taper past 5500, via bigger cam or better heads…we pretty much always made them slower. Under boost we pushed past that, I think the turbo compensated. NA with bore taper hurts top end power.

I have had more issues from freshly bore engines with new oversized pistons (and not bore with a tq plate) than a rering OEM seasoned engine.

On a low compression engine for a rering I will use moly rings. On high compression pump gas we prefer too use a cast type ring, like mentioned in that post Mike did in post 3. Or something that is both resilient and can take heat on the top ring, and a moly ring on the second. Best of both then. The cast hold up better with extra heat (like preignition) than the moly. We have had trouble with the moly rings loosing there tension more than cast rings. I have peeled some off the piston like a piece of wire. The moly rings on the top ring, can seem to work harden quicker if it is a high compression engine with bore taper. The bore taper is always the worst at the top of the cylinder were the top ring seats.

[gtobird]

Originally Posted by 400 Lemans

Check the original pistons very carefully for cracks in the ring lands. Almost sprung for a re-ring job cause the bores were still decent but after another careful inspection i found three cracks on no. 7 piston. So since i had to buy new pistons anyway, i went ahead with .030 overbore and forged pistons. Some of the cracks are really hard to see. I used a magnifying glass to spot. From now on i run octane booster and 93 gas.

You are all knowing. Yes, my #7 piston has a crack in a ring land too. How is it possible that we both had the crack on #7? Does that cylinder have weird thermal characteristics that would cause this or just dumb luck that it happened on that one for me. Then again, there might be cracks on other pistons too. When I found that one, I stopped looking closely at the others figuring I need to now go to plan B, whatever that turns out to be.

Today is my birthday, so I can't say this is much of a present. Then again, I haven't accidently hurt myself on this teardown, so I guess I'll have to accept that as the consolation prize.

OJ

[steve25]

Due to the design of the factory Q-jet Intake number 7 can tend to run lean.

It almost needs to be fitted with one step colder plug then what the rest of the cylinders can live with.

If your not looking to make over 420 or so hp this is a good reason to run the Performer Intake over a non ported stocker since it's hasa better flow balance across all its runners

[Jay S]

Happy Birthday!

FWIW, if the bores are ok plan B can be replace the problem piston with a good one and still move forward with the rering plan. I don’t know for sure, I would have to look, but I would think I would have some STD stock 67 to 75 400 pistons squirrel’d away you could have for shipping cost. The later 71 and later low compression engines don’t usually have issues with ring lands, and may be better candidates for std replacement pistons.. I had a complete set I gave to someone that needed a set of rods. I think I have another STD set somewhere.

[gtobird]

Originally Posted by Jay S

Happy Birthday!

FWIW, if the bores are ok plan B can be replace the problem piston with a good one and still move forward with the rering plan. I don’t know for sure, I would have to look, but I would think I would have some STD stock 67 to 75 400 pistons squirrel’d away you could have for shipping cost. The later 71 and later low compression engines don’t usually have issues with ring lands, and may be better candidates for std replacement pistons.. I had a complete set I gave to someone that needed a set of rods. I think I have another STD set somewhere.

That's very kind of you. Thank you. For the time being I'm going to hold off and think this through. I had similar thoughts to your suggestion. I'm trying to remember what happened to the set of standard size pistons I had from my last engine. It's so long ago, I can't remember if I tossed them out or kept them.