Rb 25 Ring Gaps

[megzz]

I'm rebuilding my RB25 with 86.5mm 9:1 comp CP Pistons. CP recommends the top ring = Bore x.0055" and the 2nd ring .004-.008 bigger than top. (where as in older thread CP recommended the top ring = .0065" or there abouts)

My question is, is CP's current recommendation safe to use to avoid the breathing issues that they were previously known for or should I still am for a tighter gap? (Will be fitting oil restrictors as well.)

My setup should see a max of 300kw with a hypergear G3 running 18-20 psi. Street and track use.

I've search over the last few days but the threads I found are few years old now and it's quiet possible that ring materials have changed so its probably time for a new one.

Cheers

[skylinecouple]

Good question. In fact it's these little details which can be the difference between a successful build or not. I'm watching.

[NYTSKY]

You can run them on the tighter side.

[33GTRV]

What fuel, boost and is there the potential of nitrous in the future. Ask yourself this, then post the answer and you will get a reasonably accurate response

[megzz]

What fuel, boost and is there the potential of nitrous in the future. Ask yourself this, then post the answer and you will get a reasonably accurate response

just 98 pump fuel. 20 psi and definitely no plans nitrous!

[33GTRV]

Ok, for turbo and pump fuel applications only I use the bore multiplied by 0053 thou measurement, PROVIDED the bore finish is suitable for the ring type. The above measurement is in inches/thou and relevant only to the supplied cp-Carrillo ductile moly combo rings generally supplied by the manufacturer. If using full moly rings or a rh8 style ring it's different again. I suggest it's gapped by someone who knows what they are doing as its a time consuming process that is easily done wrong. Pm me for further info if you like, but be warned it may take a day or two for a reply.

Cheers, allan

[r33_racer]

Its probably abit too late now, however I always run the second ring tighter then the top. Usually use bore x 0.0065" for top and bore x 0.005" for second ring gap.

Never had any issues nor any signs of butting. Having a loose second ring gap is only encouraging the gasses to get past the rings easier. That doesn't make much sense.

[megzz]

Ok, for turbo and pump fuel applications only I use the bore multiplied by 0053 thou measurement, PROVIDED the bore finish is suitable for the ring type. The above measurement is in inches/thou and relevant only to the supplied cp-Carrillo ductile moly combo rings generally supplied by the manufacturer. If using full moly rings or a rh8 style ring it's different again. I suggest it's gapped by someone who knows what they are doing as its a time consuming process that is easily done wrong. Pm me for further info if you like, but be warned it may take a day or two for a reply.

Cheers, allan

Cheers guys. I'm having a mate of mine who familiar with building engines do the gappings with me, though i just wanted to recommend him the gap sizes as he doesn't do a lot of RB's.

Another question - Is measuring the bores and numbering the pistons to suit each bore still crucial with today's quality of pistons? Do Pistons vary that much size that could potentially cause issues? Well on a car that will make around 300kw's anyway?

r33_racer: I've read a few of your post's and i already know what you are going to say!

[33GTRV]

there are alot of ways people build engines, and all have thier positives. I prefer to run a little tight for a daily drive, hence my recommendation. As for measuring, measure, dummy, measure again, cut a little measure.....see where this is going? You can never be too sure mate. Id rather spend an extra 4 hours on assembly than removing and replacing parts due to rushing it.

[SilverECR33]

I just went throught this in feb, when rebuilding my engine myself.

I tried to ask my old engine builder about this, but he didnt read my email properly..

Some mechanics build them tighter on the second, and some build them looser on the second.

I found ACL advises to make it looser on the second due to elieviate high rpm pressure buildup between the rings. so in the debate of things i rang the ring supplier (TOTAL SEAL) in the USA and spoke with them directly. as they recommend a tighter gap on the second now.

They told me that they used to advise all piston manufacturers who used their rings 15 years ago to make the second ring looser, etc but have since changed their tune due to extensive r& d testing and advised the high rpm buildup only applied to pro stock cars running at 8800rpm plus with pistons with bad pressure release vavlues or incorrectly sized ones.

they now recommend larger at the top.. then i also read through nissans RB manual and they also recommended to reduce the second..

this all confused the f*k out of me so i decided to increase the top one slightly. gap em both the same.

does nissan, acl and all the other manfucaturers use outdated info like total seal suggest for these motors/pistons?

mines running in now 900kms so far and all is going well.

if you needed a rec, if i had to do it again, id probably just set them at 0.022 for the top and 0.026 for the bottom, for peice of mind i guess.

Edited March 7, 2013 by SilverECR33

[r33_racer]

Total seal are pretty well bang on. The hype with pressure build up and ring flutter is essentially hogwash for the most part and doesn't really appear in 95% of applications, more so just the very high end people; the other 5%.

You can gap your rings almost anyway you want, looser or tighter, so long as you are in the ballpark where the manufacturer has spec'd them you will be okay.

My ring manufacturer (Aussie bloke in Melbourne) tells me to gap my rings tighter on the second FYI.

[The Dan]

There is a point that has yet to be made and that is 45% of heat in a piston escapes through the top ring into the cylinder wall.

The fact is, the top ring is the ONLY ring that seals compression gases, so anyone saying that you need to run tighter second ring gaps to decrease excessive blow-by is wrong.

The whole reason ring manufacturers recommend larger second ring gaps is so that IF the amount of gas passing the top ring is slightly more than normal, it has a better chance of getting down past the second ring. If you get a pressure build up between the two rings, the top ring can be pushed away from the cylinder wall. Remember all this is happening very fast so it doesn't take much for heat to build up quickly at high rpm.

The second ring is designed ONLY as a scraper (please read that line again for anyone who doesn't get it - ONLY A SCRAPER), although it does aid in some heat transfer and suggesting it has, or should have, any bearing on cylinder pressure is absurd. That would be the same as saying you need to decrease your oil control ring gap for the same reason.....

DON'T USE OLD INSTRUCTIONS!!!.....PERIOD!

What ring manufacturers were doing 20 years ago has no bearing on what is done today. Not even the materials are the same, not the same quality and DEFINITELY not the same finish. Engineering practices have improved markedly and customer expectations have changed.

Simple fact - Total Seal are the ONLY ring manufacturer that suggest a tighter second ring gap. I am certainly not going to tell them they are wrong for one simple reason. THEY MAKE THE RING. Not all rings are the same and you can even open a set of total seal rings and have three different manufacturers in that one box!

Are the rings the same? Total Seal might be using a Reverse Bevel and Taper Face whereas CP might be using a Taper Hook second ring.

At the end of the day there is more than just gap you need to worry about. Your engineer has to be perfect (I suggest Chilton Engineering at Woolloongabba - 07 3391 1672), your gaps have to be spot on ACCORDING TO THE RING MANUFACTURERS SPECS and you need to install them correctly. If any of these things aren't 100%, you are going to have problems so that is why you must ALWAYS follow the manufacturers specs with rings from the clean and machine to the measure and assemble.

For the record, CP didn't have blow by problems. Engineers and builders had problems. I've used nearly every brand of piston over the last 10 years and NEVER had any issue with blowby and this can be proven by the fact we never get oil burning under the top ring (above top ring = carbon etc, below top ring is like a brand new piston). This doesn't mean we don't have problems with catchcans filling and engines throwing oil all over the place but they are two separate issues.

As a general rule you want 0.005" per inch of bore on the tightest gap on these engines (considering what you want out of them)

If Total Seal want that second ring tighter, then make it 3.406" X 0.005" = 0.017" + 0.001" for good measure (0.018") and make the top between 0.018"-0.020"

If I was running a CP set, I'd be chosing about 0.018" top and 0.020" second unless you were running nitrous in which case you'd jack them up another 0.004" a piece or so.

Anyone wishing to disagree may do so, if you wish to put it in writing here, make sure you consider peak gas velocity through the top ring gap, remembering that once peak velocity has been reached, only a detonation or misfire can increase the flow of gas past the top ring by disrupting its seal. Peak gas velocity is reached well before peak rpm so any extra gas pressure cannot get through the gap you have on the top ring anyway. The top ring might have a 0.030" gap but theres only a 0.004" gap between the piston and the cylinder wall so the effective hole area is only 0.030" wide and 0.004" long.

Further reading may be done by looking up a guy named Alastair Ansell who is an expert in the field of rings. He was employed by people like Morris Motorsport to work out engine failures for them and such. Anyone who would like a couple of his articles to read is welcome to PM me and I can have them sent to you.