Piston dia vs cylinder bore.

[Cudadrag]

I am sure this question has been asked but I have not seen. How much bore space can your moly ring take up? I am looking to replace my beatup Ross pistons. The pistons are listed 4.375" When I take actual bore measurements it measure closer to 4.380" 4.378" on average.

Is .002" to .003" difference the norm? I'm sure there are rings that can support a wider gap, but what is best practice?

Reading thru tech pages opinions vary. It appears a drag race motor may want a little wider gap then say a circle track motor where operational temps are more constant.

And does ring thickness play a role? Or does a thinner more expensive ring just offer less friction?

I'm not sure we wish to add ring end gap to the discussion. This topic may need a few threads on itself.

Please feel free to think outloud.

[70AARCuda]

ring thickness is determined by the ring lands on the pistons...generally 1/16 is a good performance size ring...

forged pistons like ross need a certain amount of clearance to allow for expanation as they get hot in the bores...consult the piston manufacturer for the required clearance..

[cudabob496]

the forged JE Pistons in my Cuda use a thinner ring size, which means less friction, and more engine power output. I could have opted for thicker rings. I do see more oil consumption than normal, but who cares, if you are not fouling plugs, and get more power! A phone call to people like JE would be worth it!

[JVMopar]

Each piston manufacture specifies the amount of clearence that they would like. A good machine shop will hone each cylinder for the piston that's going to be in that hole. Infact if they will just bore and hone it to whatever I'd go to another shop. I've been there and done that. Took a block and pistons in and they asked what I wanted it bored too. I said what ever size the pistons are.

As for ring endgap. There was a recient article in circle track magazine. Total Seal Rings are the hot thing in racing. So an engine builder dyno'ed back to back rings and endgaps. They found little to no difference in rings or endgaps (of course to a limit I'm sure). Then he took the 2nd rings off of the standard rings and it made more HP than with with both rings. I've run ring end gaps of 0.030" with no problems. The diesel that I'm currently reringing has a service limit of 0.060" ring end gap for a 3.5" bore. And that's with a 21:1 compression ratio. The rings that came out of it had a 1/2" end gap and it still ran.

The biggest thing on rings is the side clearence. You want it to be tight 0.001-0.002" and no more than 0.006". When you get excessive clearence the ring flutters in the land and looses it seal with the cylinder. If the piston skirt is still good a ring groove can be resized for a spacer But you gotta find a shop to do it. I have a perfect circle regroover I got off of ebay. Doing vintage diesel engine rebuilds almost all pistons need to be regrooved but are otherwise in perfect working order.

As for bore wear I would be more concerned about taper. Something like 0.001" taper or out of round is considered the limit. For a racing engine it should be less. For a regular street service engine I've run ALOT more without problems. As for diesels I've run as much as 0.015" bore wear. I use0.010" or 0.020" oversized rings that I clearence to spec at the bottom of their travel.

So as long as your piston skirt to bore clearence is good and the taper and out of round is ok. Get some file to fit rings and clearence them for a minimum of 0.004" per inch. And you'll be good to go.

[JVMopar]

I should add that a piston is measured down on the skirt. Each manufacture has a specific place to measure. But generally it's 1/2" down from the pin and perpendicular to the pin.

[John Kunkel]

I can't imagine any set of aftermarket pistons that would vary .002"-.003" from one piston to another so I doubt the original bores were that far apart in size unless the machinist screwed up.

[cudabob496]

Hopefully cylinders are bored with a torque plate in place.

[Engine Guy]

The Clearance a Manu usually wants on the Piston, is usually built into the Piston.
As an Example,
if a Forged 4032 "Cam & Barrel" Design Piston, is "Listed" for a 4.375" Bore, generally, the Piston when Mic'd will measure around 4.371 to 4.3715, and the Rec'd clearance will be .0035" to .0040".
2618 Alloy Forgings typically are built a little smaller yet, for added clearance.
These are "Nominal" Clearance sizings only for general use, with such applications as Nitrous, Blower, or Marine requiring anywhere from .001" to .003" additional.
As Noted, Piston Sizings are usually Taken perpendicular to the Pin on the Skirt, 1/2" down from the Pin(check manu for exact measuring Point.)
As for Ring End Gaps, if that was your Question ?
4.375" plus .005" File Fit style Rings are available in the 1/16, 1/16, 3/16 Ringpack configuration.
The "extra" .005", when calculated using the Mathematical formula "PIE" which is 3.14, means those rings "mathematically" anyways, can be filed to set Ring EndGap on Bores ("5" X the 3.14) or roughly speaking .015" ABOVE the designed 4.375 Bore Diameter, albeit, they are honed to "fit" the 4.375 Cylinder Curvature.
However,
It just isn't Reccomended anywhere, "that I know of" to run "4.375" Bore Pistons/Rings in Bores THAT Sloppy !

Certainly though, I wouldn't get too stressed about running 4.375" Bore Forged Pistons in a Bore, thats getting looser towards the 4.380" Mark.
Hey, it's lived a full life, it is what it is ?
It may rattle a bit, but so what ?

Just my 2 cents
Bob out.

[Cudadrag]

Thanks for the inputs. I only asked because I may consider re-boring the next size. Has anyone bored a b-block 400 to 4.400"? I'm guessing this would be the limit. Since I am looking at replaceing all the pistons I have more bore options. It appears anything over 4.380" is custom. So are the head gaskets.

[John Kunkel]

Standard bore for a 400 is 4.340" so 4.400" is only .060" over which is a fairly common size; KB has pistons.

Because it's a common oversize doesn't mean that it's safe to bore any block that large without sonic testing first.

[cudabob496]

When I took my 440 to 496, I bored 40 over, though the machinest wanted to go 60 over. I'd rather have the stronger cylinder walls! Also, you never know if there is a flaw in a cylinder wall that might cause a failure sooner because you went to 60 over.

Also, I read that sometimes the top ring has a larger gap, so that the bypass pressure will help seat the second ring!