lyonsh

How much clearance should I have between my forged pistons and the cylinder bores on my '72 small block 350 rebuild? (bored .040 over, and I'm shooting for 10:1 compression).

The instruction sheet that came with the pistons says .0010 - .0015 inches. I think that's way too little; all the references I've seen say it should be in the .0045 - .0065 range. I seems to me that if I leave it small, I'm going to have a scuffing problem.

So to those of you who've done this more than once, what's your advice?

Noel Carboni

I built mine quite loose the last time, at 0.007" as I recall (forged Ross pistons in a 0.034" over 418 small block), on the general principle that I don't want anything to ever even come close to seizing. I hate to say it, but my brain's not working as well as it used to, and I didn't write the number down, so I could be remembering a totally bogus value.

BTW, I used "stair step" 2nd compression rings to help seal all the good pressure in. Forget the brand. Given this, I gapped the rings pretty loose too.

So far, 25K miles on this build, it's running great. :eek: I probably shouldn't type things like this... I hear Murphy reads this forum.

Bottom line is I think I'd call the piston maker and ask them whasssup.

-Noel


[Modified by Noel Carboni, 2:53 AM 2/1/2002]

norvalwilhelm

Froged pistons are in the .0045 range to .0065. I personally like to keep them on the tight side .0045. Cast pistons are another thing. They are run very tight. Are you sure you have forged pistons. If cast or forged run what the piston manufacturer recommends. If unsure call the manufacturer to check and talk to the guy doing the boring. He should know.
If you run them too loose you get rattle and the rings have a harder time do their job.
Build it tight and break it in easy. Speed shops tend to build them loose for the idiot who takes it out of the shop goes around the corner and hammers it from then on.
I even keep the bearing clearances a little on the tight side.

mountainmotor

The skirt design differs from piston to piston.Some are a little louder than others in terms of noise at start up-running while hot

If you have the L2256F .004 will be sufficient for a street car.Might want to give it 300 miles or so before hitting it hard.No multi weight oil for a while

Noel Carboni

Hey! I resemble that remark! :yesnod:

-Noel

redvetracr

lyonsh,
While i wouldn`t advise it, my race motor NEVER went as fast as it did when i had .012 piston to wall clearance. It used alot of oil but boy did it run!
....redvetracr

Jenny

resent the remark? :jester

Barry's70LT1

I very much agree with Norvalwilhelm. I have built some engines with
forged pistons and generally for street use I used .004 as the target.
When you get to the .006 - .008 range you get a noisy engine and
usually poor oil control.
I have always used TRW, and done exactly what he described and have
never had any problems.

Noel....... I recently read of some controversy regarding "gapless" 2nd
compression rings. It's about getting trapped gas pressure that passes by
the top compression ring and is stopped by the 2nd ring and that pressure
reflects back on the top ring causing a sealing issue.
I can't remember any more details.

Barry

clem zahrobsky

some of the newer forged pistons run very tight clearance so i would call the piston maker and get the straight scoop. JE pistons are made on a special machine that GM bought them to make the 502 marine pistons and these pistons use very little clearance. :chevy

inqbus383

redvetracr - speaking of using a lot of oil. My 383, set up by the previous owner, uses a quart every 300 miles. Maybe it just has really small clearances!!
:lol:

Noel Carboni

Nope. "Resemble" was the word I intended. :hat

I haven't seen any problems with the gapless rings so far, and my "loose" engine doesn't seem to have any oil control issues or other problems. Each time I put together an engine prior to this build I kept telling myself, because they always seemed to get tight when they were hot, that I would build it loose, and so I did. I'm very happy with the results.

While it doesn't get very hot because of all the other things I've done, it *NEVER* starts hard, even on a very hot day after a hard run.

-Noel

PatsLs1vette

i too run total seal rings and i love them never had a problem with them always has zero to 1 % leakdown ..and i run larger clearance on pistons too (trw forged)they make alittle noise when cold but i know ill never have a problem.

Stingy74

Aren't looser setup pistons going to slap alot upon warm up?

Noel Carboni

I honestly don't hear any real difference between my engine cold vs. hot. If they're slapping, I sure don't hear it.

-Noel

63Banshee

Depends on the piston and manufacturer. Consult the manufacturer for the proper clearance for the given piston.

Racer16k

It's probably more like .0010 - .0015 per inch of bore. 4 inch bore X .0010 = .004. 4 inch bore X .0015 = .006. Good rule of thumb anyway.