This seems to be something of a preference type of question because I have gotten different responses from my machinist, as well as tech lines I have called.

I was checking my ring gaps for a .030 over 383 I'm building, and the top rings gapped at .014 (with one at .013) each, and the second rings gapped at an even .020 each.

These are Speed Pro rings part # R-8902-30. The top ring is chrome moly and the pistons are forged 10.5(approx) Speed Pros.

My machinist told me that .014 is fine if I'm not running nitrous, and that they will open some more after so many miles are on the engine. I plan for it to be a decently strong street engine. He recommended not to go any lower than .012 on the top rings.

The tech lines I called gave me a figure of no lower than .018 on the top rings. The other tech line told me .004 (huh?).

My machinist has the best reputation in town here and I tend to prefer to go with his advice, but I'm wondering if I should just file the rings up a thousandth or so to about .016 which he recommended in case of nitrous might be in the engine's future. I sure appreciate any advice.

 

What kind of pistons do you have? It seems the hypereutectic pistons require something different than forged ones.

 

The pistons I have are Speed Pro forged part# L2491F .030 over.

 

OK, good. I would go with what your machanist says.

 

That would be .004" per inch of bore. In a 4" bore it would work out to .012.

I would say go with the ring manufacturers specs.

BigBlockk

Later.....

 

Thanks David and BigBlockk for your replies.

Ahh... that would explain what the one tech line rep meant by .004.

As an English major, my unsteady math skills dictate that a 4.030 bore's gap should be .01612.

This gives me a bit over .002 from my machinist's blessing on what I already have.

 

If you have any possibility of installing nitrous in the future, open the top to .018-.020 and the second ring to .020-.022.
The extra eng gap will not hurt. I open mine up a bit for safety.

 

It would be .016

 

Got it! And thanks again to all

 

You may want to check and see if that forging has any interference between the pin bosses and the crank counterweights at the BDC position. Over many years of using that particular part number we've encountered this problem with many aftermarket shafts. Even on the ones "profiled" for the 5.700" rods.

As I said, on numerous builds we've had to machine the lower pin bosses down to make it all fit.

Contrary to what the "industry" recommends, I would send the rings out the door at .018" top and .014" second. If you plan on "spraying" it, you can add another .004" top and bottom.
Thanks, Gary in N.Y.

P.S. If the measurement, (the thickness) between the lower pin hole and the bottom of the boss on the piston is approximately .350", you MAY have an issue. Be careful!

 

I took me a few readings of your post and I think I understand. Do I measure from the bottom of the pin hole to the portion of the piston directly under it to include the metal in between? I can't find my dial caliper for some reason, but I'm measuring it at a little over 1/8 inch. Sorry if I seem a little dense here.

 

I was just filing fitting my rings a few weeks ago and it is recommended to have the 2nd ring gap larger then the top ring. You do NOT want to trap any compression between the top and 2nd ring thus the increased gap on the second ring.
I was working with a 4.5 bore and went .026 on the top and about .028 on the 2nd. They claim that up to .032 does not affect power or leakdown.

 

I wonder if I would be okay with going up to about .017 or .018 on the tops and leaving the second rings at .020 where they are now?

 

Hello Gary, thanks again for your advice. I measured the bottom of the hole where the pin is pressed in, and my dial caliper measures .209" from the bottom of the hole to the bottom of the piston portion below it. The other side of the piston reads .199". Does that sound okay to you?

 

Like I said .016"! WHERE did I get .012? Oh well.

BigBlockk

Later.....

 

I knew what you meant. The formula you provided shed light on the subject for me.

 

I just pulled the chart on ring gaps and this is for a 4.00 bore

Moderate performance .016-.018
Drag racing .018-.020
Nitrous street .020-.022
Nitrous drag .028-.030
Turbo/supercharged .024-.026


Also a specail note
Most of the second ring gap reecommendations are larger then the top ring. Recent testing has shown a larger second ring gap tends to increase top ring stability allowing for better seal.

The gap for the second ring is .002 more then the top ring

 

Excellent! Thank you very much. My application would definitely fall under "moderate performance". Have a great weekend.

 

I really don't understand the 2 dimensions you're reading, but there should be no issue with the piston/crank. Sounds like you'll be good to go. Double check it (the clearance) between the pin bosses and the counterweights at BDC when you pre-assemble all the components. Thanks, Gary in N.Y.

P.S. As to the ring gaps, you'll be fine there also. We've done much testing (all on the dyno) and don't really buy into the rings "lifting" theory on most of these street units. Possibly in other type units, but not here. All our N/A street units here leave with .018"/.014". I did just recently deliver a stock 396, Winter's heads, with .032"/.030" gaps and the unit came off at 508HP, pump-gas, with absolutely no ring issues, blow-by, or noticeable cylinder leakage whatsoever. Unit runs fine with those gaps! This was a test for our own purposes. These were "el-cheapo" Sealed-Power E232K rings.

 

Thanks Gary, I just got back from work and got your message here. I think I understand now about the bosses and I'll do a thorough clearance check during pre-assembly. I plan to do that tomorrow.

By the way, I'm not even going to mention the different torque specs I'm getting from separate sources for my ARP main bolts! ARP told me 70/70 on the four bolt mains if I use ARP moly lube and 85/85 if I use motor oil. Someone told me 50 on the ARP rod bolts. I didn't realize how much interpretation I would be doing on these engine specs, but it is rather fascinating. Thanks again.