Bore size for Speed Pro forged with Duroshield coating

[Pentroof]

The little card that came with the L2291F pistons uses some confusing language:

"Refer to the specifications printed on the piston set, part number label for the recommended final bore size...boring the engine to the final bore size indicated will result in proper piston to cylinder wall clearance"

The label doesn't specify a final bore size, but gives a skirt size as well as a suggested minimum clearance.
skirt size: 4.07825"  +/- .00025"
min. clearance .0015"

The card in the box also goes on to state "The piston skirt size prior to coating remains unchanged. However, the diameter of the piston increased due to the addition of the Duroshield coating. Do not measure the skirt and compensate for the increase in diameter due to the coating, as this could lead to excessive clearances and piston failure."

If I measured the skirt, why would I add anything to begin with, since the coating is included in that number? I haven't measured the skirts yet, but why wouldn't they be what is reflected as the skirt size on the label?
Skirt size remains unchanged prior to coating? What are they assuming that I'm assuming? I didn't sneak in and measure them before they were coated. WTF?

I just need to know what to hone the final bore to, and I don't like the .0015"

[Pentroof]

Measurements at lower skirt are dead on with the skirt dimension listed. I think I'll have the machinist hone to 4.080" and get this thing moving.

[Joe-JDC]

A call or text to Speed Pro would clear it up for you.  Usually the piston is made for the bore size asked for, such as +.020, +.030, or a specific bore size such as 4.145".  Clearances are in the piston from the manufacturer.  Joe-JDC

[Pentroof]

A call or text to Speed Pro would clear it up for you.  Usually the piston is made for the bore size asked for, such as +.020, +.030, or a specific bore size such as 4.145".  Clearances are in the piston from the manufacturer.  Joe-JDC

Yes, I know know Joe. That's why all the extra babble seems unnecessary.

[cjshaker]

min. clearance .0015"

Minimum clearance, on a forged piston...at .0015?? Were these pistons shipped on April Fools Day? What am I missing here? There's no way that can be right?

[KMcCullah]

They must mean .0015 clearance between the skirt and cylinder wall. Which would be .003 total clearance.

[Pentroof]

Yes, .0015" min. is correct. This alloy has high silicon content and the oil return is a stock style full slot. The skirt doesn't see as much heat by conduction and the alloy doesn't grow as much as conventional forgings.

Tech line af FM confirmed 4.080 to 4.0815

[Pentroof]

They must mean .0015 clearance between the skirt and cylinder wall. Which would be .003 total clearance.

Nope. That is the minimum bore clearance.

[cjshaker]

Apparently I'm out of touch, because that just seems like a recipe for disaster.

[FB]

yeah Doug i'm with ya on this ..hell a .001 clearance is like a press fit, at .0015 you'd have trouble getting the piston in the bore.
i'd do .003 or even .004 thou clearance over mic'ed skirt size

[Pentroof]

Many discussions across multiple forums and multiple engine types about this topic and these pistons.

http://speedtalk.com/forum/viewtopic.php?t=41022

[Pentroof]

Bulletin from FM in 2013

[FB]

tell ya what, take a piston put it in the oven at 250 deg for a half hour, take it out and immediately  mic the skirt.
that will tell you if they are full of crapola.
if there was that much controversy over a type of piston, i would go buy something else and sell the others

[My427stang]

4032 pistons and similar alloys like the Speed-pro are nothing like 2618 or whatever the old TRWs and the like were. 12-16% silicon, they don't grow nearly as much.  Secondarily, it's also a function of bore size

A coated high silicone piston at a .0015 thousandth MINIMUM doesn't scare me, although I'd likely round up to .002 or .0025 if it was going to run hard now and then. 

Look at bore size here, they show a .002 min for for a 4.20 bore, and it's certainly a high performance piston. 

https://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc4032.pdf

I get some engine building rags too both online and hard copy, an the consensus seems to be that with those types of coated pistons, when coated, you ignore it's there and emasure.  Meaning, your .0015 clearance if you cleaned off the coating, would likely be more like .0025

I'd also be careful thinking that an oven heated piston indicates what's happening.  Yes a piston will grow with heat, even at 160-200 degrees, but the block grows too, and a a hole gets bigger with heat. 

Last though, remember, those aren't true hipo pistons, they are decent forged street pistons.  I like them when they are appropriate and they work nice, but can't compare them to a 2618 race piston.  Where a 2618 would grow and be quiet, those would rattle in the bores if put at old school clearances

[FB]

I hear ya...still don't like .0015, the PDF says .002-.0025, like almost double .001-.0015.
jus  sayin i know the growth will be in the block too, but i also know what a hunk of any aluminum does when you introduce heat, coating or not
the alum is still getting hot and growing at a different rate than the block, do i know more than the manufacturer....NO, but my pucker level is
telling me to ronald reagan it....trust but verify.