Forged Piston clearances.
 

Hi all,

My Scooby had a bit of a tick/knock so we ripped the engine apart to find this...

So my crank case is all split and is currently sat at a friend of a friends machine shop. My car is a JDM Bugeye STI and had forged pistons as standard, can tell by the Hf-1 stamped on the back, so naturally i want to stick to having forged pistons.

I've been looking at getting Forged Mahle 4032 Pistons which i can get in 92/92.5/93mm sizes but i've been reading about thermal expansion and that the 4032 alloy needs a 3 thou running clearance.

Using 92.5 as an example, is that running clearance included in that 92.5mm piston size, so bore to 92.5mm or does my machine shop have to bore out to 92.50762 (3 thou = 0.0762).

I'm sure they'll know all this but i just wanted to ask or my own knowledge really.

Cheers!

Karl

Might as well order the 4032 92.5 stroker pistons and get a 2.5 crank whilst it's in bits.

I'd love to but i'm keeping the costs as low as possible due to moving out soon, obviously without sacrificing quality. The re-bore and (possible) crank regrind aren't actually costing me anything due to exchange of favors, so I'm more than happy with just having the car back up and running. I've had it almost 2 years and'ts probably only been on the road about 3 months of that in total. :lol1:

They should measure the pistons and then machine the bore to get the required clearance.

Pistons have the clearance, so you would bore to 92.50mm, as already said the pistons should be measured, then the bores machined. 4032 material is lower expansion than the usual 2618 forged piston material. I would run them at .0025" clearance if you are not going over 400, or .0030" for 400-450.

I see, well i'll be buying the pistons this week so i can give them to the shop for correct measurements. i'm aiming for 400+ so a .0030" clearance as you said.

Again just for my understanding, if i get the 92.5 pistons and i need a 0.003" clearance, does that mean they will open the cylinder up to 92.50762mm (assuming they measure exactly 92.5mm) or am i completely miss-understanding this?

No, the clearance is on the piston, the bore would be 92.50mm and the pistons are 92.43mm approx.

Ahh i see, so the pistons have the standard clearance in the size of .0025" so i need to mention that i need a .0030" clearance at which point the will bore .0005" larger to accommodate this. Got ya! Thank you for clarifying this.

I should be around 420 so is it worth going for 3 thou even though it's only 20 over what you've said?