Forged Piston to Wall Clearance

[95REGF150]

I am posting this in the newbie forum because I haven't hit my post count yet.

Got my 7 bolt block at the machine shop right now doing a 2.3 build with stock 2.4 crank, eagle h-beams, and wiseco pistons with the skirt coating. I am planning on running around 25psi of boost from my FP green.

Talking it over with my machinist and he said after calling wiseco they recommended adding .001" of wall clearance for boost over 20psi. With their starting recommendation for these pistons being 0.003". That would bring the total wall clearance to .004" which to me seems high. This motor will see quite a bit of street time and I dont want to have excessive piston slap if I dont have too. I would be more comfortable with 0.0035" I think.

What are you guys running for piston to wall clearance with over 20psi boost on forged pistons?

 

[totaleclipse_05]

I've never built a motor so maybe I'm completely wrong, but don't forged pistons expand a bit once warmed up?

 

[jery1234]

.004 is the correct clearance for that boost level .

 

[95REGF150]

.004 is the correct clearance for that boost level .

Ok thanks, haven't actually done one of these motors before but I come from the V-8 world and usually its only .001" - 0.0015" for every inch of bore size.

I've never built a motor so maybe I'm completely wrong, but don't forged pistons expand a bit once warmed up?

And yes forged pistons have a different thermal expansion coefficient than the originals. So at startup you will definitely get some piston slap until they warm up. However, they will continue to expand under the heat of boost more than they will during NA driving and so you open it up even more for that. I just didnt want to open it up too far or you can get pistons slap when driving around NA and not in boost.

 

[lasthope05]

If you are shooting .004" total clearance you need to machine it to .0035". The skirt coatings take up .0005" and once worn down after initial startup/break-in you will have the clearance you want. However I would always shoot for looser. I like .0045".005"

 

[miliman13]

Those #'s are solid. No need to be concerned.

 

[donniekak]

You'll be fine. A big difference between the v8's and where the dsm lives is power per cylinder. A dsm making 100hp per hole is not even close to something special around here, and considered a really mild build that probably would be fine on a stock bottom end.

 

[Kapok6]

.001" and your motor isn't going to last. That was a horrible recommendation by your machinist.

 

[95REGF150]

Thanks for the responses guys. We will stick with the .004" wall clearance.

.001" and your motor isn't going to last. That was a horrible recommendation by your machinist.

He did not recommend .001" clearance total. It was recommended that the .001" be added to Wiseco's starting spec of .003" for a total of .004" for boost over 20psi. The specs that come with the pistons are very vague. They just show for mild street spec and race spec but make no mention of whether that is NA or boosted. He just called Wiseco to confirm and that's what they recommended for boost over 20psi.

 

[miliman13]

^ the rest of us, understood your first statement.

Good luck.