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Old 06-04-2011, 11:42 PM   #1
AaronGTR
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Piston ring sizing

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 they sent me... 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?
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Last edited by AaronGTR; 06-05-2011 at 12:47 AM.
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Old 06-05-2011, 09:07 AM   #2
plastic_indian
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Given that this is a blower motor, I personally would not be as concerned with a slightly open top ring - in fact, I might go with a greater gap than spec on purpose. It is logical to assume those piston tops will be dealing with a bit more heat than a NA motor and a subsequent increased risk of butting the ends and locking the ring in the bore. When in doubt, however, follow the piston manufacturer's recommendations rather than any stock spec - I've seen some unusual ring gap specs even on relatively plain-jane hypereutectics. Case in point, I built a small block a few years ago with a set of SilvoLites; the top ring gap calculated to about .030" - and that was for a normally aspirated build!

Or if it still bothers you, get a set of Total Seal rings made for your application.
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I'm pretty sure you don't need 600HP to merge safely onto a freeway. and if you do, you're doing it wrong.
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how do you think i ever got girls to show up? i'm only paying for so many of them
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Old 06-05-2011, 10:06 AM   #3
AaronGTR
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Interesting. Well the pistons are sealed power, but the rings are Perfect Circle brand from Mahle. They were both made specific for this engine though and neither came with specific ring specs. The rings just had some basic installation instructions with the formula of .004" for every 1" bore diameter, but said to follow the engine manufacturers specs. Guess I'll have to do some more digging. Even with the SC, combustion temps are only up during hard driving. Rest of the time coolant temps etc are normal. I'm worried about losing HP yes, but I also don't want excessive blow by when driving around normally since this is going to be my daily driver again. I was thinking of calling the engine shop and asking for their input as well, but they won't be open until tomorrow. Which means I can't start putting it together today. More setbacks.
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Old 06-05-2011, 10:40 AM   #4
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Excessive blowby would be my biggest worry as well.

I recently replaced an engine in a SC'ed Cobalt because it kept blowing the dipstick out of the tube under boost. Compression readings were pretty close in the 200 - 220 range but a leakdown test found as much as 60% leakage past the rings.

I'm no engine rebuilding expert so I don't have any input other than I'd be calling for some more advice.
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Old 06-05-2011, 09:01 PM   #5
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I stand by my suggestion to consult the *piston* manufacturer for application-specific end gap recommendations - the slugs are the greater mass in the reciprocating assembly and will therefore have the greater impact on the thermal characteristics of the whole assembly thereof. I acknowledge the blowby concerns, and I certainly respect a dealer line tech's experience with blowby in blown OE applications. In reference to blowby concerns, I suspect blowby will be minimal (ie, within the capacity of a properly functioning PCV system to manage) under low load conditions, whereas blowby will be the least of concerns if those periodic "hard driving" conditions bust a top ring land.

In response to Scott's experience, I have to inquire about the driving habits and any, ahem, *modifications* pertinent to said LSJ replacement; in other words, I would strongly suspect abuse or owner-induced damage was at fault in a 60% (!) blowby circumstance before I would be concerned about GM's ring gap specs.

Again, if you want to have your cake and eat it too (and Aaron has rarely spared an expense so far), get Total Seal to make a set of rings to solve your problems. After all, GM spent hundreds of hours ironing out every detail of any particular engine's build sheet; a couple weeks waiting for custom parts is a small tariff to pay for combined performance and reliability in such a modified application.
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I'm pretty sure you don't need 600HP to merge safely onto a freeway. and if you do, you're doing it wrong.
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how do you think i ever got girls to show up? i'm only paying for so many of them
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Old 06-05-2011, 09:17 PM   #6
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Yeah. Problem is I don't have a few more weeks. I've already been without this car for months and am already weeks past when I thought it would be done and back together. My registration is expiring in a couple weeks for both my cars. I need to get the grand am running again so I can sell the grand prix. I'm not paying another years registration on it just to keep it for a few more weeks 'cause as soon as the GA is running, the GP is getting sold. Also because I moved a few weeks ago and I still need to change my address with my car insurance company, and I want to drop the coverage on the GP at the same time. There are many reasons, but I need to have it running in about 2 1/2 weeks, so I can't wait for another set of rings. I'm just going to follow the best gap recommendations I can find and put it together.
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Old 06-05-2011, 09:36 PM   #7
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**** man, if the slug manufacturer can't give you guidance, and you're dead-set on bolting this thing together this week, then follow the generic 125% figure for boosted ring gaps, watch the timing, and pray to the car gods for leniency.
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Quote:
Originally Posted by cardude007617 View Post
I'm pretty sure you don't need 600HP to merge safely onto a freeway. and if you do, you're doing it wrong.
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how do you think i ever got girls to show up? i'm only paying for so many of them
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Old 06-05-2011, 11:02 PM   #8
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Quote:
Originally Posted by plastic_indian View Post
In response to Scott's experience, I have to inquire about the driving habits and any, ahem, *modifications* pertinent to said LSJ replacement; in other words, I would strongly suspect abuse or owner-induced damage was at fault in a 60% (!) blowby circumstance before I would be concerned about GM's ring gap specs.
Yeah, I know. The whole set of circumstances surrounding that motor replacement were just weird.

Cliffnotes: Said LSJ gets traded in. New owner buys as used car and has dipstick problem. Notices issue before leaving Northern Texas. Takes it to dealership which removes GM approved GM Stage 2 kit (SC pulley, injectors, ECM cal.) and replaces two cylinder sleeves along w/ pistons and rings.

Customer drives to Austin (Home?). Comes in for dipstick issue again. I find borderline compression and high blowby. Engine replacement authorized because of recent work done. Problem solved.

Yes the car in question is a heavily modified POS that somebody beat the absolute dog**** out of and dropped at the first sign of a big problem. Buuuut we couldn't prove a damn thing other than the engine had been worked on. Sad thing is... 2,000 miles later, the new owner puts the car in a curb totalling damages of $5k+... may total the car I JUST replaced the engine in....
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Old 06-05-2011, 11:15 PM   #9
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^^I hope at least you got paid realtime instead of warranty time. As far as totaling the mess so soon after major work, what else did you expect? Nobody ever totals a car that comes in for an alternator, it's always the one you lose your ass on that ends up balled up tight by an ungrateful owner two weeks after you bleed over it...
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Quote:
Originally Posted by cardude007617 View Post
I'm pretty sure you don't need 600HP to merge safely onto a freeway. and if you do, you're doing it wrong.
Quote:
Originally Posted by bricooper78 View Post
how do you think i ever got girls to show up? i'm only paying for so many of them
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Old 06-06-2011, 12:15 PM   #10
Malaclypse
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Quote:
Originally Posted by plastic_indian View Post
^^I hope at least you got paid realtime instead of warranty time. As far as totaling the mess so soon after major work, what else did you expect? Nobody ever totals a car that comes in for an alternator, it's always the one you lose your ass on that ends up balled up tight by an ungrateful owner two weeks after you bleed over it...
Nope... got hosed for warranty time. Lost my ass, too.

its not really wadded up... just damaged enough that the cost to repair may exceed the car's value. Being an '06 with 54k on it doesn't help. Cobalts don't hold their value.
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Old 06-06-2011, 10:11 PM   #11
AaronGTR
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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.
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See it here. the total package.
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