nonhog

Just got off the phone with my machinist and he asked me did whoever set this up plan on a heavy shot of Nitrous? I said I don't i bought it as is.
Well as my luck would have it whoever bore and honed the block set it up
with 4 thousands clearance not 2.
My machinist said it'll work but it won't last and it will be noisey.
I told him to hold off and I will pop the heads of my other 455.
maybe its standard still and he can bore/hone it the way it should be.

my luck !

cutlassefi

What pistons?

nonhog

Oh you want the whole story! Sorry my bad, forged 2278P

cutlassefi

.004 isn't too much for virtually any forged piston, cast? yes, forged No.

However some guys are taking their clearance measurements off the skirt size, even when they're coated. The coating wears off, you are supposed to bore the block .030 over, not .032 or .033 etc. The clearance is built into the piston.

But with that said the new KB Icon piston series has the std bore of a 455 listed as 4.126 and they recommend .0045 clearance, which is supposed to be built in as usual. But guess what? If you bore it from the true std size you'll be .001 shy on the clearance. So your finished size needs to be 4.156 on a .030 over, not 4.155.

.004 on a forged piston should be fine, and not overly noisy. Maybe he just wanted to sell you another set of pistons.

One other question, is he using a torque plate to hone it?

nonhog

I'll be the 1st to admit I'm no expert. He said it may(probably) make noise.
Especially cold starts and it would be fine but he mentioned the ring life may suffer.
He never once mentioned new pistons.
So would you run it or bring in my other block?

cutlassefi

Would he be boring the other block perpendicular to the mains and honing it with a torque plate? If not don't bother and run what you have. I'm sure you have other things that aren't quite right as well.

Warhead

I agree with Mark.
These pistons MUST have at least .004 (I like more), if you don't, you will score the hell out of the pistons.

I have a set of L2390F's that have .0065 clearance, and when I pulled them out of the block, they looked like new. They went right back in.

Jim

88 coupe

The "true standard size" was 4.126".

The extra thousandth made it a 455 instead of a 454.

Norm

nonhog

Good questions, "other thing?" Like what?

Any piston slap?

ziff396

I have 2323F Speed Pro's in mine. .005 piston to wall. No slap noticed. This is what my engine builder recemended with these pistons. They where coated.

BlackGold

If the previous engine builder had used just .002 clearance, your post today would've read like this:
"Why is my new rebuild overheating? And why doesn't it have any power?"

cutlassefi

Well are your bearing clearances as recommended? Did you check the big ends of the rods? Check line bore? Deck warp? Etc.

And IMO .005 clearance on the 2323 Speed Pros may start to be a bit much. They have a lot of silicon in them, meaning they don't grow much.

The KB Icons are made from 2618 which grows alot more and they recommend only .0045, FWIW.

64Rocket

If it was mine .004 would be the minimum I would run. Otherwise you WILL score the skirts to a point you will need new ones. On well abused engines that race more than drive I use .005 or more. I run .006 in my engine and it has been in there for the last seven years.
I would have them just scratch hone and send her home.

Gene

Run to Rund

I have found with the TRW/Speed Pro stock replacement forged design pistons, a minimum of 0.0045" at the bottom of the skirt is the minimum for reliablity, no skirt scuffing. On the "select fit" W30s the factory used D size bores with A size cast stock pistons for .0025-.0035" skirt clearance. See page 6B-24 of the 1971 service manual.

nonhog

Well got off the phone this morning and my machinist and he said its at
4 thou. now and he said he'd like to re-do the hone.
Then he says the pistons are forged but they are not a race piston and they spec @ 2 thou.
I could not find specs anywhere on the net as they are really old (but unused)
He will do whatever I tell him. Still thinking?

anybody have old speed pro specs for 2278p pistons?

Run to Rund

The TRW stock type replacement forged pistons I have used, such as L2320 for W31, L2214NF for 425, and L2323 for 455 all needed more than .004" clearance to the cylinder wall, measured at the bottom of the skirt. As i posted above, a minimum of .0045 and .005+ is fine. They were spec'ed on the sheet from TRW/Speed Pro for .0015-.002" but the skirts will swell and scuff badly with that clearance, even with .0025 or .003". I think you need a new machinist as you are not doing a tractor rebuild and that is all he knows.

Run to Rund

L2278F is listed as a Chevy race piston, Speed Pro number 7028P, top and second rings 1/16", oil ring 1/8", compression height 1.51 which is .1" shorter than an Olds 400. It has .927" pins and Speed Pro sheet that came with their pistons in 1976 specifies a minimum .005" skirt clearance across the thrust faces at pin centerline. Six people including myself have recommended at least .004" and more is better. Where you measure matters too as the skirts are barrel shaped on most of these old design pistons. I prefer to measure near/at the bottom of the skirt, where the piston should be biggest (unliess heat has collapsed the skirts).

Again, this was a RACE piston back in the 1970s and was intended to have extra clearance. Even the street designs I noted above needed more than .0025" at the bottom of the skirt, and that would be more like .0035" or more at the pin centerline. the Olds pistons have .060" pin offset which "holds" the piston against the major thrust side of the wall. The Chevy pistons generally have 0 offset but still want extra clearance.

Run to Rund

I also don't understand how you gave us a 4" bore piston part number for a 4.126" bore 455 that with stock rods needs a 1.74" compression height. Something isn't right. Are you sure of the brand and part number you gave us?

nonhog

#'s came off the piston, thats all I have to go with. Again these are old,
maybe 20 years? I thought maybe I noted the # wrong but there are threads online with that # going in 455.

I know this machinist and he's done Olds before in fact a couple of guys from ROP have used him. He is no stranger to engine building.
Has a real good reputation and is an avid racer.
Thanks to all for your input!

cutlassefi

I'll jump in here. Any idea why the skirt is smaller at 90 deg to the pin than the bottom of the skirt? Cuz there's more heat there, you may only have .0015 or so clearance at the bottom of the skirt on a new piston, that's to keep the pistons stable when cold etc, reduces slap. Plus the coating Speed Pro uses is designed to wear off. Read their website, it says for scuff free break ins.

You're almost never supposed to measure for recommended clearance there, almost always at the pin axis. More piston rock from more clearance, more ring wear.

IMO run an extra .001 or 2 and let it go at that, .005 at the bottom of the skirt is too much.