piston sizes

[spyderdrifter]

So I finally got my 0.040 over pistons last night and I brought them to work along with the old stock pistons and connecting rods. I'm punching out the wrist pins so I can give the rods and new pistons to the machine shop after work. Any how, I decided just to compare the pistons to each other and they honestly don't look different in diameter. There is a noticeable difference, but not much. Due to this I looked up specs for both and the stock are supposed to be 85mm and the 0.040 over 86mm. I measured each with a digital caliper and got 84.46mm for the stock one and 85.24 for the new one. Are these correct numbers for their sizes? I know they won't be the 85 & 86mm (respectively) due to needing room for rings in the cylinders, but over half a mm smaller than the bore? Is that right?

[BogusSVO]

Did you measure on the skirt, 90* to the wrist pin center line?

How to Measure, Mic, a Piston

[spyderdrifter]

No, I measured at the top of the dome. Didn't know where else to measure. Guess I was wrong.

[BogusSVO]

Yes you measured wrong. We all have to learn.

Measure again and post back what you find.

[spyderdrifter]

Yep, we all do learn Which spot am I supposed to measure though for the diameter? I'm not sure by that spec sheet.

Wait, is the diameter determined by measuring 90° of wrist pin but on the skirt itself?

[BogusSVO]

Yes measure on the skirt, that is the widest point of the piston.

The piston in the "How to" I linked is not a DSM piston, just an example so you can see where to measure.

[spyderdrifter]

Sweet. I'll measure again here in a little while and let you know. Thanks.

Much better numbers. Got 84.96mm for stock, and 85.94mm for the new. Thanks for your help.

[BogusSVO]

86mm x .03937 = 3.3858 This is your finished bore size

85.94mm x .03937 = 3.38345 This is the true piston size

So 3.386 - 3.3835 = .0025 PTW This is what you should end up with for PTW

Boosted, then they were not .040 over, either boxed wrong, or were measured wrong

STD bore size is 85mm x .03937 = 3.3465

.040 = 1mm (.03937)
so your 86.5mm would be .060 over

[spyderdrifter]

So are you saying they're not 0.040 over? They are stamped with 0.040 over markings.

[BogusSVO]

Spyder... The last part of my post was for BoostedTsi

But the top part was for you.

yes you have 1mm/.040 pistons, the math proves that.

[Bo0sTdTsI]

Thanks for that bogus i actually did not know it that way, thanks alot for the classification, im glad im still young lol

[spyderdrifter]

Sounds good. Didn't know he posted either.