Ring Gaps

[33GTRV]

Ok guys, ive built a few engines over the years but have never used a mahle, CP or ross piston. I currently have a set of custom CP's from a stroker im going to be building and i want to know peoples views on ring gaps for these pistons and rings as ive heard a bit of a mixed debate on thier recommended clearances bieng too big. Current bore/piston set is 86.5. I know the "General" rule of thumb is 5thou per inch so that would land me somewhere in the 17 thou or so. What do the builders lurking out here run and what is thier recommendation? Hoping to crank around 700-800hp and reasonable boost out of it, mainly street but occasional track so i was thinking around 20 thou for the top, 24 for the second and 16-17 thou for the oil rings.

Any input is appreciated.

Cheers, Allan

[GTRNUR]

The data to calculate the propper ring gap is in the CP pamphlet that comes with the pistons.

There is a multiplier that is used to calculate the correct clearences for each of the ring packs which is based on the bore diameter and the application of the engine.

The multipliers are based on the known thermal expansion that the rings will undergo during combustion.

Different piston manufacturers use different ring materials, so the multipliers will be different for all ring manufacturers. If you are using special rings like Total Seal rings, then use their specifications and not CP's.

A similar rule applies to the piston to bore clearences. Many people belive 0.003" is correct, but this is not true. The engines purpose combined with bore size and piston composition, are the factors that determine clearences.

If you really want to make your head hurt, think of what piston coatings do to the equation. Using coatings to make the pistons run cooler means they will expand less, and as a result you get more blow by as they don't expand to seal the cylinder bore 100% correctly. Since nobody has a means of knowing the piston crown temperature reduction, there is no real way to be sure of how much tigher you can make clearences.

If you haven't filed rings before, go very slowly. You will quickly learn how the rate material grinds off the rings works with each different type of ring. Each of the rings (top, middle and oil control) will file at different rates due to material and thickness. You can't add material back if you go to far.

It might be overkill, but I also measure the ring gap in the bore with the ring sitting at the height that it will be when installed on the piston at TDC. Just use a vernier caliper with the adjustment lockout to set the ring height from the deck.

[33GTRV]

measuring the ring gap at tdc is the way i was taught. Im aware of the dramas with totalseal and moly filled ect. Just thought i might have gotten lucky with previous experiences from people on here. Most of what i build os big cube forced V8 stuff and 75% of the time its Keith blacks and totalseals, cant really go wrong with them The recommended gaps are BoreX.0055 for turbocharged/nitrous fed engines from CP's website(inches) If thats the case its 3.406X.0055 which comes up at just under 19 thou or .47mm. But like i said, there has been some debate over the accuracy and experiences people and builders have had with these. Im just looking for a bit of clarification

Cheers, Allan

[abr33]

I did mine the other day, cp's, 86.5mm bore. top ring should be 18.7 thou (19) second should be 23 to 28 thou. i had an oopsie and did a top ring about a thou over and a second ring on a different cylinder at 27 thou meant to be 24 but its within tolerance. easy to get carried away. i used internal mic's and checked bores they are spot on and i sat ring 5mm under and square to deck. im not running huge power but about 300 heading to about 350kw.

REMEMBER, nitrous needs more gap, i also believe if you run ethanol you can run a tighter ring as it runs alot colder. (can anyone confirm??)

[abr33]

And what files do most people use? i used a little diamond file

[33GTRV]

i have made up a jig for my dremel and use a fine disc and depending on the ring i use a paste to stop the chipping that can sometimes occur to the plasme coating in some rings. Its pretty much a cheap version of a proper ring grinding machine.

[abr33]

Oh cool. i just did my best with a little diamond file. one did have a micro chip come off it but nothing much. dont think it'll matter though

[abr33]

Do you think it'll matter?

[SiR_RB]

like above the CP spec sheet should tell you clearences for street, drag/race, and high boost/NOS applications....

but its all in inches so you'l have to convert to mm.

little diamond hand files work a charm

[The Dan]

Ring gaps are not quite as important as they used to be. Ring design and improvements in material mean there is a much wider tolerance allowable and as long as you don't run under the usual minimum, then you will be ok. Of course going too large can be a problem but 0.007 per inch will still give you no blowby whatsoever on any of these jap engines. Some people will tell you that the rings need to be gapped wider because you are going to make 800hp but in reality the temperatures are controlled to a certain degree in the tune at these levels as I would like to assume you will be running individual cylinder EGT's.

0.005 per inch still runs true for the manufacturers you mentioned. We use all of those brands and they are all the same for minimum gap requirements. Always add another 0.001 per inch for every 150 shot of nitrous it will see to be safe. Usually on anything that is going to see a decent amount of hp above factory levels on an 86.5mm bore I run 0.020 top and 0.002 more on the second ring minimum. This gap doesn't mean a lot, just make sure its 0.002 more at least than the top gap. Don't worry about your oil control rings, they will be something like 0.040 from what I see on most. Do not ever touch the expander and you never usually have to touch the rails

0.020 and 0.024 are perfect for what you are doing, even if it was going to see Nitrous up to 150 shot

[33GTRV]

Legendary, thanks for the info mate. I ended up going 20 and 24-25 and yes, the oil rings were a little on the "Larger" side but i didnt touch them. As for nitrous, im not a real big fan, but i have been known to venture into the dark sometimes

Thanks again, Allan

[JustinP]

Yes thanks for this info, very timely as I'm just about to do mine for the first time too. I'm also using 86.5mm CP pistons. I bought a relatively cheap ring grinding tool. Hope it will be ok!

Legendary, thanks for the info mate. I ended up going 20 and 24-25 and yes, the oil rings were a little on the "Larger" side but i didnt touch them. As for nitrous, im not a real big fan, but i have been known to venture into the dark sometimes

Thanks again, Allan

Edited December 29, 2011 by JustinP

[T04GTR]

the gaps from cp are shit. blowby city.

if you are going to revv it heaps yopu must have the second ring a good 5 thou looser.

i have done 10 on the top 15 on the second with good results. that is with nitrided chrome rings.

fyi. stock nissan vg30dett gaps are 9 on the top and 25 on the second...

[T04GTR]

Legendary, thanks for the info mate. I ended up going 20 and 24-25 and yes, the oil rings were a little on the "Larger" side but i didnt touch them. As for nitrous, im not a real big fan, but i have been known to venture into the dark sometimes

Thanks again, Allan

that is gonna breath alot.

10 and 15 with 28psi, 50,000km empty catch can...

[The Dan]

I've done CP's at 18 and 20 without an issue whatsoever. Even with 26-28 psi.

28psi with a normal healthy 15-20% leakdown past the rings is A LOT of pressure/flow of air into the crankcase. Managing the pressure is the only hard part. We are working on a solution currently that will virtually eliminate all other oil control problems

In your above post you did say a 5thou gap apart from top to second. He is running 20 top and 24-25 second. This is still around 5thou difference. TBH its always better to go slightly bigger on the second ring but it won't stop the blowby. 10thou and 15thou are pretty tight. I would want to see some evidence of a high boost, high hp application and see the gaps myself to agree but I'm always keen to learn new things.

Bore hone has a great deal to do with how the rings will bed in as well. Are you sure the breathing engine you might have seen was not breathing heavily due to another reason?

[NoFigJamHere]

To the original poster: What i consider equally important as the gap is that you make sure the end gaps are square! I have never seen a hand filed gapped ring that sits perfect in the bore.. And as for issues with excessive leak down/blow-by refer to first statement.. Ive seen engines built by "so called guru's" that had .020" on inner edge and .030" on block touching edge... If u can beg/steal or borrow a proper ring filer. And another thing when the ring is in the bore it MUST be perfectly square. Best thing to use is any flat top or center dished piston you can fit down ur bore with an old top or 2nd ring in the oil control groove. Trust me this is the only way you'll get ring square. Hope this helps and good bud.

[33GTRV]

Yeah, i actually have the tool for squaring rings, and im pretty pedantic about squaring the ring edges. Im not actually new to the task, just new to the brand of piston. Like i mentioned in the original post, i build alot of V* garbage with KB's and JE's, and know thier rings and the way they react to different situ's well, just there was a bit of debate over the CP range of rings and thier recommended gaps in the past. Its good to get some first hand knowledge from the guys who build them for a living(RB's) I find the best way to check for square filing is to use a high intensity LED light and your eyes. I place a sheet of paper over the light, squeeze the ring closed and look for the gaps, pretty easy to see them lol. From there you just gently touch them up and get the desired gap

[abr33]

i found doing the gaps square hard by hand, got em pretty good except 1 had the right gap at the bore, and prob 4 or 5 thou (at a guess) on the inside, however that will be covered by piston right? im not going as crazy with power as you but

[NoFigJamHere]

To abr33: if your saying youve only got 5-6 thou gap on the inside how do u get 10,15 or 20 thou feelers between the gap? a little too much is better than slightly not enuff...

[abr33]

Sorry, bad explanation. it woulda been prob 3 thou but i mean 3 thou larger not 3 thou gap