piston ring sizing

[AaronGTR]

Anyone know anything about it? New bores in my block are perfect. .04" oversize, and I have matching pistons and .04" oversize rings as well.

Stock specs for ring gap are .006"-.014" for the top ring, which seem on the tight side to me. It is a thinner ring but they still need to leave room for thermal expansion. General rule is .004" for every 1" of bore diameter, and 3.66" would give me .01464" ring gap, but they also say go by whatever the manual says. Problem is the rings I got... both the top and second rings are right at .018" in the bore, with no filing done yet. So the top rings are already .004" OVER the requirement! The spec for the second ring is .0197-.028", so I can file the rings and still be on the tight end of that spec and be fine. Same with the oil rings. They say.0098-.05" gap (which is a HUGE tolerance IMO), and mine are at .035" so I probably won't even touch them.

I'm really just worried about that top ring. I don't know if .004" over is acceptable or if that's going to make me lose some compression and power. If so, I don't know what I can do about it. They don't sell oem style rings larger than .04 oversize. If I could get some .06" oversize I could then file them down to spec. I can only get that size for aftermarket pistons though. No idea if they'd be the right size for my ring grooves. Any ideas? Anyone know where I can get some .06" oversize tops rings that are the right thickness etc to work with oem/sealed power pistons?


I was also doing some measuring getting ready to put the crank back in. Keep in mind I'm not using a super expensive accurate gauge, but it is a dial caliper gauge and should be fairly accurate. Not sure how much affect the heat and humidity here right now might affect measurements as well. My main bearing bores are at 2.842-2.845" and spec is 2.8407-2.8412, so it's a few thousandths on the loose side, but the caps weren't fully torqued so they might get a tiny bit tighter. The crank main journals were 2.634 on avg and spec is 2.6473-2.6483". Rod journals are 1.992 and spec is 1.9987-1.9994. Rod bearing bores are 2.128-2.132 and spec is 2.124-2.125.

So basically tolerances are all on the loose side, but mostly by about one hundredth to a few thousandths. On one hand I'm thinking that's good and I shouldn't have to worry about any spun bearings, and maybe it will just spin nice and freely and make good power. On the other hand I'm worried it may be too loose. I did buy a new oil pump, and I shimmed the bypass spring about 1/8" to raise the peak allowed pressure a bit, so hopefully I'll have enough oil pressure/flow to deal with the loose tolerance. Maybe it would be a good idea to run a thicker oil though? Like 10w30 or 10w40?
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[trotterlg]

If having .018 inches of top ring end gap is your biggest worry then you are in great shape. If you figure the area that air has to get past those rings you will see that it is extremely small, your engine will never know the difference. Larry

[SappySE107]

You have to account for your supercharger, and .004 is a generic rule for stock rebuilds. I am building a motor right now that has .019-.021 gap on the top ring and .030 2nd ring for a high compression NA motor. The second ring has to be larger than the first for gap and I have read tests showing no significant power change from upping the gap. Might have been in engine builder magazine anywhere from 2-4 years ago so I only remember the results of the tests. I know they tested a gap that was a lot more than stock, double or more even, and the results were minimal in change.

[AaronGTR]

I was going off this article for information as far as installing the rings. http://www.circletrack.com/howto/1818/index.html Seemed like it had some good info. Didn't say much as far as differences in gap sizing for different applications, but he did make it sound like proper gap size was critical which is what has me worried. I'm just trying to find out for sure what is an appropriate gap for my application and if .004" over size from the max recommended is ok. Especially since the formula would only call for .001" over stock specs, and I don't know exactly how much extra thermal expansion I'm going to get.


Here is one article that claims that the ring side clearance in the piston groove is more important than the end gap. Any thoughts on that? http://www.ringspacers.com/tips.htm If that's the case then I should be ok since I have brand new pistons and rings, and I checked the side gap clearance and it's good. Of course... they claim that worn ring grooves are the primary cause of blow by, and on total seals tech page they claim that ring end gap is the primary cause of blow by. So is there anyone with some real trustworthy info out there? Or is everyone just trying to sell me something?

[trotterlg]

This is how it is: Too little gap = distructed engine, too much gap = nothing bad. Larry

[dskopek]

Most modern rings use .004" per inch of bore as a standard, .0045" per inch of bore for blown or turboed. As Ben said above the 2nd gap needs to be larger then the top so you do not trap combustion gas between the rings becouse that will cause them to flutter for lack of a better word and lose the seal with the cyl wall. The same thing is true of the ring grooves in the pistons, they HAVE to be straight and true or the ring can flutter and not hold a seal to the cyl wall. I saved a tech paper that covered this really good along with dyno numbers and leak down tests, I will find it and post it, good reading if nothing else.

[AaronGTR]

sent a tech question to Mahle and got a response from them. They said .018 was fine for the top ring, but to make the 2nd ring .022, and to leave the oil rails at .035".

I told them I had a supercharger too, but I'm not sure if they accounted for that or not. Someone sent me a chart from another ring manufacturer that had rules for figuring gap based on bore and usage (ie. nitrous or turbo, street, drag ,or race, etc). Standard .004 x my bore of 3.66 would be .015 rounded up, but .006xbore was recommended for boosted and would be .022". If I'm at .018 now maybe I should go in between and make them .020"? And do the same with the second ring? I'm still not sure. Maybe I should do .005 x bore, but that would be .0183 for the top ring so it's basically there already. The specs they gave me would equal .005xbore for the top ring and .006xbore for the second.

[trotterlg]

I am fairly sure you are way over working (worrying) the problem. Larry

[AaronGTR]

lol, well maybe so... but I only got one shot at this and I don't want to screw it up. I can't afford to tear down this engine again if something's not right.