JE piston size discrepancy

[AC sports]

Last year I ordered some forged JE'S supposedly for an 84.5mm bore.
Spec sheet said:
bore size 84.5mm
Set clearance to 0.0762 mm

My pistons however measured at 13 mm up on the skirt, at 20*c, with a calibrated mitutoyo 0.001mm micrometer measure 84.398 mm

The math doesn't add up.
If I set my bore to 84.5mm as stated, Ill get 0.102 mm clearance.

They are 2618 alloy.
The 0.0762 mm (3') recommendation may be minimum....but with an 84.5mm bore 0.102mm (4') would rattle on warm up I'm told.

What are people's thoughts?

[DSRE388]

Thats about a .001" difference, id personally run it

[BillK]

My pistons however measured at 13 mm up on the skirt,

Is that where the instructions say to measure them ? I personally would not worry about it. The extra .001 will be good insurance if you run it hard. The specified clearance is almost always a "minimum" clearance suggestion. If you want you could call JE and ask them but I would bet they will tell you the same thing.

[Circlotron]

Thats about a .001" difference, id personally run it

No, it's about .004" difference.

[digger]

Thats about a .001" difference, id personally run it

No, it's about .004" difference.

0.1016mm = 0.004"

But that's not the difference

[makin chips]

Thats about a .001" difference, id personally run it

No, it's about .004" difference.

Which is only .0005" larger than the high end of their recommended minimum clearance of .0030 to .0035" for that bore size. Something like 2.500" to 3.625" IIRC from looking at the pdf earlier. I posted this earlier, but convinced myself I had miscalculated something and it wasn't right so I deleted the post lol.

2618 JE piston pdf

[AC sports]

So the consensus is run them @ 0.004' piston to bore?
I don't want them to rattle when cold though. That's what I'm concerned about and also JE's claims they measure 0.001' more than I'm miking them at.
I spoke to them. They claim they left there at 84.42mm +_ 0.0127mm.
My freshly calibrated micron resolution mitutoyo at 20*c half inch up the skirt gives a consistent 83.397- 83.398 mm

[KnightEngines]

Run em at .004" , or .0035, or .0045"
You'll never know the difference.
Doubt they'd rattle noticeably at .005.
If they do, then that's the price of running forged pistons, they'll shut up when they get a little heat in them.

I don't even really look at the bore size spec for pistons, I mic them at the size point, look at min clearance spec & decide from there how much they need given the usage, I often end up settling on .0005"-.001" more than min recommended.

[David Redszus]

Do the pistons have a skirt coating?

[AC sports]

No, they don't have a coating...but I've just sent them off to get a dry film skirt coating done. Then the plan is to run them at 0.004' measured before the coating.
The coating should wear to fit the bore I'm lead to believe. These are for a 3.327' bore remember.
Thoughts?

[gmrocket]

They are probably measured at JE with a non contact laser mic....no way you'll get as accurate a measurement

Trust what they say

Finish your bore at 84.5 which will give you the recommended 003" (.0762mm)

[engineguyBill]

As others have stated, call JE. They produce quality products and they will take the time necessary to help you with your inquiry.

[leahymtsps]

As others have stated, call JE. They produce quality products and they will take the time necessary to help you with your inquiry.

Bill o/p already posted this
I spoke to them. They claim they left there at 84.42mm +_ 0.0127mm.
My freshly calibrated micron resolution mitutoyo at 20*c half inch up the skirt gives a consistent 83.397- 83.398 mm

[4vpc]

If it's possible to put them to one side for another job i'd be doing that. Coating will only build up the skirt, what about the top ringland area?
Also, how accurate is your bore? Was it done with a deck plate? The more accurate you are, the tighter you can get it that's one of the reasons they quote big clearances.
Live and learn, next time bore to the piston you have in your hand, not what is written on the box, that's my machinists rule and we do it every time. Then there is no excuse for a mistake whatsoever. He's bored a bit tight before and i've had good grounds to reject it so he's done it again with no quibble.

As far as 'ring JE up', well you don't know who you're going to get to talk to and there are some pretty useless people there who pick up the phone.
They haven't done any R&D with this particular engine and will just give you the standard JE 'one size fits all' type sheet they put in with every piston set.

[jsgarage]

I ran 0.004" piston clearance for decades on the street with forged 2618 alloy. They do rattle a bit on startup in my block but it only lasts a minute or so until the forgings heat up and expand to size. Makes me smile when the universal experts in the parking lot tell each other, 'its his radical solid-lifter cam'. In fact the engine has a mild grind hydraulic cam....