I'm getting ready to gap my mahle rings.Doing a 355 NA (might run a 150 max shot now and then).I was looking at the spec sheet Top ring gap .018 NA (.0045x4.030) 2nd ring gap .022 NA (.0055x4.030) .If running 150 shot .024 top and .030 2nd.

1 what would be the gap tolerance +.002/-.000 ?
2 old school says smaller 2nd ring gap is this no longer true?Whats better?
3 .006 differance between NA and 150 nitrous,Might run small shot at the
track should I run the bigger gaps?How much will this hurt if I never run gas?
4 do the gaps look ok? Or should they be changed at all?
5 any advice you have to offer.
6 THANKS

 

How far down is your top ring? What width ring?

 

Yep larger 2nd ring gap. Put the gaps in there that the manufacture says and going too big on gaps is not a bad thing.

Bret

 

top land is .093 Ring thickness is .058 x .149 width.

Bret I think what your saying is bigger is safer.On a leak down test how much of a loss do you think there would be between .018 top /.024 2nd vs .024 top /.030 2nd ?When you do gaps how do you balance out between running NA and maybe a shot of nitrous.I know we can't have every thing.

 

I'm in a similar situation, but going to run a small-medium boost blower on my 383, and I have Mahle pistons with the metric rings like you do. I called the tech at Mahle and discussed it and he said I should run a gap in between the NA and Blower gaps listed on that sheet that comes with the pistons. I haven't yet decided if I am going to do that or go larger.

With Nitrous, it might need to be gapped for worse case, cause when you're on the gas, it don't matter that you run around 99.9% of the time not on the gas.

 

do you have a normal set of specs that you go by? like top ring

.150 down bore 5/16 ring gets .0xx gap
.250 down bore 5/16 ring gets .0xx gap
.300 down bore 5/16 ring gets .0xx gap

what do you usualy look for in an engine running a 150shot with his rings
Ring thickness is .058 x .149 width

 

why the larger 2nd gap now?

 

Thinking on piston ring gaps has also changed. In the old days, second ring gap specs were tighter than those for top rings because they didn't see as much heat. But this didn't account for inter-ring gas pressure buildup between the top and second rings. If the pressure between these rings equals or exceeds the pressure above the top ring, it can cause the top ring to lift off the bottom of the piston ring groove and lose contact with the sealing surfaces. It also inhibits the ring's ability to transfer heat from the piston. To keep inter-ring pressure from becoming a problem, the current trend is to create an easy escape path for the built-up pressure by gapping the second ring larger than the top ring. Another benefit is that because gas pressure is now directed downward toward the sump, any oil that has collected in the ring pack areas will go with it.

ps this is copied from car carft

 

Ring gap is HUGLY affected by piston material and overall design. ALWAYS go by what the piston manufacturer says, not what the ring manufacturer says. You can thow all the old "rules of thumb" in the trash. They'll get you in more trouble these days than they'll help.

Too big a gap is always preferable to a gap that's too small- destroy a motor or two from butting the rings closed under detaonation (as I have several times) and you won't think going smaller is such a good idea. If you look at the actual exposed open area of a ring gap (piston-to-wall gap X ring gap), it's a pin-*****. Open it up from .025 to .030 and it's just a slightly larger pin *****.

 

listen to him!

FWIW I know a few guys who found more power with MORE ring gap and not less even though the rings were not butting together.

Bret

 

Dam good explanation

 

I generally go by the manufacturers recommendation.
I also don't run the ring "down" the piston like some do.Ya don't get a "clean" burn like that.
I have an engine know that is running a 300 shot with the rings .120 down where a N/A engine is.
I generally check their spects with mine
street build=.005 top x bore
.0055 2nd x bore
No2=.0065 top x bore
.007 2nd x bore.

Having said that my last two used gapless top ring and they seem to really work good. They are $300.00 a set and most people won't pay that much unless it's a trailer queen.
I also don't use narrow rings in a performance build.I use 1/16 x 1/16 x 3/16 with standard tension unless it's a trailer queen then I will use .043 top or run just 2 rings but that set up doesn't last long with just two rings.
If the compression get's past the top ring you're done anyway.

 

Piston MATERIAL affects ring gap as much as it's design, like I said above. Having done some research on this subject myself it appears that thermal conductivity of the piston material plays as much of a role in ring gap as the design of the piston does (how far down the top ring is from the face of the piston).

Coefficient of expansion of the material actually plays a very small role in all this, believe it or not. How the piston design/material manages heat is the real key.

Example:

Keith Black hypereutectic pistons have very low thermal conductivity. That's the biggest reason you can run tight piston to cylinder clearance, but also the reason they require MASSIVE top ring gaps. All the heat stays up on the top of the piston- from the top ring up- very little gets conducted down the body and into the skirts.

Keith Black forged pistons have a very high thermal conductivity. You have to run much larger piston to cylinder clearance becuase the whole piston gets hot and expands as the face of the piston heats up. However, this also helps conducts heat away from the top ring so you don't need to run a very large ring gap.

 

Going by Sealed Powers recommendation, I ended up with an .024" top gap, and .026" second gap. This is with Ross 4.030" bore nitrous pistons for my 355.

Frank

 

Nothing to do with heat anymore than it did 15 or 20 years ago. It is just a case of engineers getting smarter. If engineers (20 years ago), knew all there was to know, they would be unemployed today. This holds true with any race team engineers in service today as well. This is why race cars slowly get faster as time goes by. Any engineer that proclaims to have all the answers is either lying or not very smart. Cheating notwithstanding, the faster cars are not without engineering screwups, they just have fewer screwups. This is what progress is about. Enuf ranting.

This in not new ring technology. Look at your 10+ year old GM service manual. Some ring mfrs. have been recommending larger 2nd ring gaps longer than that.