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Old 05-28-2010, 06:50 AM
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Chevy 400 ring end gaps

Hi everyone. I'm in the process of assembling a SB 400 with SpeedPro H615CP pistons and SpeedPro moly rings. I'm finding my end gaps to be .020 to .022 on the top ring and .018 to .020 on the second ring. I know Hypers usually require more gap than regular cast pistons so does this sound about right? Too much, too little? This engine is built for street performance and won't see 6000 rpm. I know alot of you guys have insight and experience in this area, I would sure appreciate your input...thanks in advance....

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Old 05-28-2010, 08:45 AM
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Chevy 400 ring end gaps

my opinion
i'd use steel rings for daily use no drag race
moly rings are for tear down after so many races -motor's .
i like 18 gap on the top and 16 on the second if it's new bore and honed block .
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Old 05-28-2010, 09:12 AM
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Quote:
Originally Posted by moretork
Hi everyone. I'm in the process of assembling a SB 400 with SpeedPro H615CP pistons and SpeedPro moly rings. I'm finding my end gaps to be .020 to .022 on the top ring and .018 to .020 on the second ring. I know Hypers usually require more gap than regular cast pistons so does this sound about right? Too much, too little? This engine is built for street performance and won't see 6000 rpm. I know alot of you guys have insight and experience in this area, I would sure appreciate your input...thanks in advance....
I've only ever built three 400's (not counting the one in my shop now), so I'm no expert on them. What I did in all three cases was to use the ring manufacturer's upper limit recommendations for an engine that will be run hard, which was 0.0045" to 0.005" per inch of bore diameter for the top ring and 0.005 to 0.0055" per inch of bore diameter for the second ring. I went w/0.021" top, 0.023" second.

If the build is milder, this can be reduced by 0.001" per inch of bore diameter (0.0035 to 0.004" top, 0.004 to 0.0045" second). Some rings are spec'ed to be gapped at 0.003" per inch of bore, so there are variations between the different makers and types of rings.

This is only if there are no available factory recommendations. There are specs available for your exact parts online if you don't have them with your parts. Also, if the HU pistons you have require something different, use their recommendations that would have been w/the pistons when they were bought or can be found at their site.

A note- better to "err" on the wider side than too narrow.

Good luck.
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Old 05-28-2010, 01:54 PM
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moretork....just to clarify, there are cast rings, moly rings and plasma moly rings. I've been using 5/64 moly rings in my street engines for a long time and NEVER had a problem. 1/16 low tension plasma moly rings are for the race track. Go with what Cobalt says on the ring gap....good info.
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Old 05-28-2010, 02:16 PM
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Steel top compression rings are mainly a diesel/boosted/nitrous-type thing. The OEM's have also started using them or so I've heard, but sparingly by the big 3, possibly more so w/the imports?

All the rings I use are some type of cast (ductile or gray) iron, usually w/a moly inlay. The "premium" nitrous rings like A&C and Hellfire's are basically a type of ductile iron, so I'm thinking steel rings in a "normal" street/performance application is going to be rare.

I've also installed plasma moly rings before (also cast)- they are (AFAIK) still on the road, this was years ago.

Last edited by cobalt327; 05-28-2010 at 02:22 PM.
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Old 05-28-2010, 05:08 PM
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Thanks guys for your input. The only info sheet with the rings explained orienting the rings,etc. Thanks Cobalt, seems like good advice,will file it for next build.....thanks again...
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Old 05-28-2010, 06:16 PM
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O.K., before I finish here, I checked another cylinder. Top ring .024, 2nd .020. The bore is .040 over, but this seems excessive to me. Am I going to get blowby or low manifold vacuum? Or am I just getting ahead of myself here. I just want to build it right and not have any problems down the road. Just for the record, it is a street engine but I will certainly beat on it now and then, will maybe see a little strip action...is there any downside to the extra gap?
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Old 05-28-2010, 06:36 PM
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Did the pistons say anything about the ring gap?

The 1974 Chevrolet Overhaul Manual calls for the gap to not exceed 0.020" That's in a stock engine.

It is thought by some builders and ring makers that a slightly wider second ring gap helps the first ring to work better, but you know what? I believe you might be over thinking this, and that you will find no excessive blow-by w/the gaps as you have them. In that the rings you have installed, have the gaps they do, these rings may well NOT adhere to the "larger second ring gap" theory, or whatever you want to call it.

All three of the 400's I built, were 0.021" top, 0.023" second and they were fine regarding blow-by. That said, there are members here who really like the 400 and probably build more 400's in a month than I have built total- maybe they will offer you some additional advice.
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Old 05-28-2010, 07:43 PM
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I would run with what you have unles you want to take the time to enlarge the second ring gap larger. Even at .025" top it doesn't tremendously bother me. I could run with what you have, the top ring is going to close up more due to heat.
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Old 06-07-2010, 06:16 AM
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Well, I've finished assembling the short-block, the average top ring gap was .022, 2nd .020. I think you're right, I'm over-thinking this. At least I won't have to worry about butting ends together. Thanks, everyone, for your input, I really appreciate it...MT
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