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Discussion Starter #1
Am installing the piston rings on my 351C but am coming out with some numbers that are causing me some worry.
First engine I've ever built so not keen on what tolerances you can you slip by with but heres what I'm looking at.

It's a 351Cleveland bored to 4.03 (I'm running forged pistons). So I bought a pair of rings for a 4.03 bore off of summit, E251K .030 is the set the machine shop told me to buy. As I install these rings i'm getting numbers that are a little off from tolerances that are in my service manual and am not sure whether I need to worry.

Book says compression rings can be anywhere from .01-.02. Here's a chart of what the feeler gauge was giving me when I set the rings in the cylinder.

P1- P2- P3- P4- P5- P6- P7- P8-
Top Ring - .019 .022 .021 .018 .021 .020 .020 .023
Bottom ring- .022 .020 .022 .020 .022 .022 .020 .022

Now obviously some of these are out of tolerance from the standard book range .020 maximum.

So what do I do at this point, I'm not sure if machine shop is to blame or rings are sized wrong, or both?
Thanks.
 

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What's the piston manufacturer recommend for your intended use, one specified gap doesn't work for all applications. A wider gap is better than too narrow of gap, you can run a much wider gap without any ill affects than you would guess.
 

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Discussion Starter #4
Not exactly sure as far as manufacturer goes, because any info or directions that came in the box was useless. They're Sealed Power brand, maybe I'll give them a call on monday to hopefully find some possible specs?

But if Allens right then maybe I'll just build her up as is.
 

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Not exactly sure as far as manufacturer goes, because any info or directions that came in the box was useless. They're Sealed Power brand, maybe I'll give them a call on monday to hopefully find some possible specs?

But if Allens right then maybe I'll just build her up as is.
You can file the ones under but the ones over being max .003 over I wouldn't sweat. What HP are you trying to achieve? What is the goal for the engine? Street or serious track machine?

Allen
 

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Discussion Starter #6
My first engine so not really aiming for hell of a lot of horsepower, just a street car with a bigger cam and a couple little aftermarket things.

I've got ring filer but honestly wasn't even worried about the ones that fell a little low of .02.
Think i'm good to just put it together then?
 

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Do not be concerned with the ring gaps that slightly wider than the book spec--NO WORRIES THERE.

Probably not an issue with the couple that are slightly tight either but unless there are no piston ring instructions in the box from the manufacturer of the rings, I'd slightly clearance the couple that you think are tight and then put it together. There will be Zero (0.000) effect in HP, oil usage, or engine longevity.
 

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If you're worried about the ones that are lower than .020 take them and try them in the cylinders where you came up with the two largest numbers......and measure again.
 

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Discussion Starter #10
Yeah so what I plan on doing is just taking any upper compression rings that are smaller then .02 and file them to .02. Then I'll take any of my lower comp rings that are .020 and file them to .022-.023 just so that I have lower rings that are larger then upper.

For any that are really off, like my top rings for cylinders 8 and 4 I'll swap around so that hopefully I can close in some gaps as long as the problems are with the rings and not the cylinder. If there .001 off here and there I guess I just won't really worry about it.
Gunna have some fun trying to not break the rings while getting them off the pistons today...

Thanks for the replies guys, appreciate it.
 

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I always liked to check the gaps in their respective positions in their respective cylinders. Hopefully all of your cylinders are the same bore. As was mentioned. a little loose is better than not. Some mfgs. recommend a smaller tolerance for the top ring compared to the second. Be very slow with a ring filer, they can remove metal very quickly.
 

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Ja, 'close in' ? ALL of those gaps look on the tight side, to me. I'd want .026, minimum. Tight ring gaps are NOT good, or helpful. LSG
 

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Discussion Starter #13
LSG;
I don't have any manufacturer specified end gap values, so I've been going off of the Cleveland build books values and the values I've seen off line.
The build book specifies between .01-.02 for both top comp ring and the lower comp ring. From values I've seen offline and the article provided by 64 1/2, .02 for top rings seems to allow for optimal blowby as long as the lower ring has a larger end gap. Maybe I'll go to .024 on the lowers to be safe as far as butting concerns.
 

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According to the experts the 2nd ring doesn't see as much heat because it's farther from the combustion chamber, so the larger gap in the second ring isn't for expansion, it's so blow by from the top ring doesn't get trapped between 1st and 2nd rings and cause ring flutter.
 

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Don’t assume anything ring gap is crucial to longevity and performance forged pistons have a different gap setting than cast pistons just like the Keith black hypereutectic are opened up more then the forged piston call the manufacturer and set the gaps to there recommendation don’t take any chances on a new build
 

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Ja, ring end gap is not changed from forged to cast, what makes the difference is how close to the top of the piston is the ring groove. KB hyperuetechtic pistons have the grooves closer to the top, and require larger gaps. What book have you got suggesting .010~.020 gaps ? Thats just plain wrong, burn that book ! .010 is a disaster waiting for a chance to happen ! I would call .020 the MINIMUM gap, there is no maximum. Run .026, or more, on the top rings, and run .030, or more, on the 2nd rings. Going as tight as possible is not a wise move. LSG
 

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We're all assuming you know how to check ring end gaps, right? Ring in multiple places (top and bottom) in the cylinder and "squared" using a tool or inverted piston? What does your piston mfr recommend? Usually, for a "street" engine, top ring at .0045" x bore, scraper at .005" x bore so your top ring gap would be .018+/- and scraper .020+/-.
 

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Ja.mc44,
Ring end gap has been misunderstood for decades. MOST believe it has a dramatic affect on leak down ( an air check of ring seal) , but it has a very small affect. "Perimeter" ring seal is where big leak down numbers come from , not end gap. In the '60s TRW was a popular , reasonable price forged piston. We would run gaps of .016 on the top and .012 on the second ( which is NOT a compression ring , it is an oil scraper ring) . Around 1980 aftermarket pistons got to the point where we had to increase the "Ring factor" ( bore times .004) to bore times .005 or .020 gap for a 4.000-4.030 bore and we went to .016 for the second. Piston ring manufacturers began recommending larger second ring gaps as a direct result of testing done by Nascar engine builders where "ring flutter" ( top ring) was reduced when the second ring gap was increased.
The "current" recommendation for "naturally aspirated" street performance is .020 top and .022-.024 for the second. Rings DO expand with heat and the gap shrinks BUT they MUST NOT touch on the ends. Doing this makes the ring try to stop the piston in the bore and further heats the ring expanding even more. This eventually destroys the ring breaks the piston ring lands and deposits metal in the piston skirts.
Check your rings carefully and use the gaps I recommended. Blowby will not be an issue.
Randy
 
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