# What clearance do you run on Mahle pistons?



## Norms68 (Jul 17, 2013)

Hey guys,

Started my 461 build. What do you run for piston to wall clearance on your Mahle pistons? I'm using 4.155 bore.

Mahle website says .0025 - .0033 (seems little tight)

Butler Performance says .0035 - .0040

Any thoughts?

Norm


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## geeteeohguy (Feb 2, 2008)

Based on your info, if it were me, I'd set the clearance at .0033. Close enough to Butler, and within factory specs. Just a thought.......


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## BearGFR (Aug 25, 2008)

What type are they.... cast? forged?

What kind of shape is on the bottom edge of the bottom oil ring land.. Stepped? Radiused?

What do you plan to use the engine for?

--- it all matters ---

Bear


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## geeteeohguy (Feb 2, 2008)

It's a 461. Obviously, he plans on using the engine to raise he!!, like someone _else_ I know on this forum.....


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## Norms68 (Jul 17, 2013)

BearGFR said:


> What type are they.... cast? forged?
> 
> What kind of shape is on the bottom edge of the bottom oil ring land.. Stepped? Radiused?
> 
> ...


Mahle Power Pak Forged 4.155" Flat top.

Beveled bottom edge.

Street/strip approximately 500hp.

The reason I asked was that I was measuring my piston diameter at 4.1516" + or - .0001" and if I go with the standard bore of 4.155" it would put me outside of the factory clearance of .0025 - .0033" 

That's when I called Butler Performance and they said to use .0035 - .0040" which would put me in the sweet spot for 4.155" bore.

Norm


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## BearGFR (Aug 25, 2008)

All things considered, I'd go with the recommendation of the piston manufacturer. After all, they should know the characteristics of the metal alloy they used to make them and how it's influenced by their manufacturing process.

In your case, I'd lean towards the .0033 because that's on the loose side of the range Mahle recommended. 

Factory specs for Pontiac don't apply - forged pistons need more clearance than cast ones do.

(Guilty as charged, Mr. GeeTee sir...  )

Bear


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## Norms68 (Jul 17, 2013)

BearGFR said:


> All things considered, I'd go with the recommendation of the piston manufacturer. After all, they should know the characteristics of the metal alloy they used to make them and how it's influenced by their manufacturing process.
> 
> In your case, I'd lean towards the .0033 because that's on the loose side of the range Mahle recommended.
> 
> ...


Now I'm confused because you say lean towards factory specs then you say factory specs don't apply for Pontiacs and forged needs more clearance lol.


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## BearGFR (Aug 25, 2008)

Sorry, didn't mean to confuse you. When I said factory specs I meant _Pontiac_ factory specs, as in original manufacturer. I didn't mean Mahle... 

Bear


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## Norms68 (Jul 17, 2013)

Ok so recommend .0033 and I'll give Mahle a call also just to make sure I don't need to run more room on them with the specs of my build. Thanks.


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## BearGFR (Aug 25, 2008)

Now that sounds like a plan, get confirmation just in case that printed doc is incorrect. Heck, if they reiterate the numbers you have, I'd not be above asking them a question like, "Ok thanks. Can you think of a reason other engine builders like Butler Pontiac might make a different recommendation?" It's all in how you approach people.

Bear


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