# piston to wall clearance



## Chris-Austria (Dec 23, 2010)

Hi!

What piston to wall clearance works best for Pontiacs with forged pistons?
I have a set of Ross pistons (less than 1% silicone)

They call for about 0.004" for street and 0.005" for modified engines.
My wall clearance will be 0.0045" to 0.005", that was measured by the shop.

Do you think they will make a noise when cold? If not, what do you think is the max. clearance for my setup? Because they call 0.004" minimum.
Would you increase the clearance just by lets say 0.001"? (428 stroked to 462, roller valvetrain, maybe 500hp)

Here is a picture of the ones I got. to me they look very nice:











Chris


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## geeteeohguy (Feb 2, 2008)

I think you are fine where you're at. I would not go with any additional clearance. You appear to be right in the ballpark.


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## Chris-Austria (Dec 23, 2010)

I'm just confused because of the term "minimum". This sounds like a little more would be better but my old pistons needed about 0.003" but they had I believe 21% silicone.

The new ones seem to be stronger and need more clearance... but at what clearance will they start to make noise when cold?


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## PontiacJim (Dec 29, 2012)

Chris-Austria said:


> I'm just confused because of the term "minimum". This sounds like a little more would be better but my old pistons needed about 0.003" but they had I believe 21% silicone.
> 
> The new ones seem to be stronger and need more clearance... but at what clearance will they start to make noise when cold?


There you go overthinking it again.:nono: As the pistons get hot, they expand. Different materials expand at different rates. The minimum clearances are just that. If the clearance is too tight, your piston will expand and do damage to the bore/piston, or even seize in the bore. 

Your maximum clearance is for a gas only engine while you will see that nitrous or turbo adds more clearance because they get hotter and expand more. I would not worry too much about piston slap if you are within the limits of the clearances your instructions tell you to use. From what I have read, piston slap only occurs when the engine is first started up and cold, and seems to be common/normal with forged pistons -if you get any piston slap at all.

If you have concerns, I would email Butler and ask them if you will experience any piston slap when the engine is cold using the clearances you have.:thumbsup:


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## Chris-Austria (Dec 23, 2010)

I'll do that, thanks 
I was just wondering what they mean by "minimum" but now I understand. I'll always have the minimum clearance because this is build into the pistons.. I ordered for a 4.160" bore and on the box it says bore 4.160" and net size 4.156". In reality we have measured the bores again and they are between 4.1605" and 4.161". So no piston clearance will be more than 0.005" and no one will be less than the 0.004" they want. I guess I really do overthink things, but I want to know as much as possible about this stuff 

I also read that the less silicone the piston has, the more it will expand, so that explains the need for more clearance if you increase material strenght by reducing the amount of silicone. It's nice to know how things work, thanks!


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## 666bbl (Apr 13, 2014)

My old racer ran with .008, 4.351 bore, naturally aspirated (1050 Dominator), 14.1:1, Speed Pro/TRW pistons. Noise? Yes, and never exceeded 1500 RPM until the coolant temp gauge moved. There used to be a formula as to the dimension/bore size, as in the bigger the bore the more room needed. You have to be true to yourself before you decide. Will you lean on it as hard as I did mine? Doubtful, as I tripped the lights at 7800 RPM in good air but built it for that. Compression ratio, bore thickness, power adders, all can add a thousanth here n there.


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## Chris-Austria (Dec 23, 2010)

I won't use power adders and I won't race it except 1-2 times a quarter mile every year. I added a oil temperature sensor, so I won't rev it above 2000rpm until it warms up... Maximum rpm may be 6000 but I guess it won't see more than 5500 most of the time. CR is 10.32:1 and bore size 4.160". Butler said it will make about 500hp and 500ft lbs. I cannot tell if this would be a "modified engine" because Ross says minimum clearance for modified engines is 0.005". There are some modifications, but nothing too extreme I think.


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## BearGFR (Aug 25, 2008)

Set the clearances where Ross recommends. They know their own product. Don't worry about them making a little noise on a cold engine, we don't build these things because we want peace and quiet, right?

Bear


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## Chris-Austria (Dec 23, 2010)

You're right. Butler said 0.005" will be okay.
We'll see if it makes a noise when cold, probably not.. most important is that it will be okay even with a little noise when cold.
Merry Christmas to all


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## PontiacJim (Dec 29, 2012)

Chris-Austria said:


> You're right. Butler said 0.005" will be okay.
> We'll see if it makes a noise when cold, probably not.. most important is that it will be okay even with a little noise when cold.
> Merry Christmas to all


Chris, if you can hear the pistons slap when its running, you've got the wrong exhaust system!:thumbsup: The only thing you should *hear* is that loud Pontiac rumble; the only thing you should *smell* is the fine fragrance of exhaust fumes; the only thing you should *feel* is the vibrations that engine is going to put out; and the only thing you should *see* is that big grin on your face in the rear view mirror!:lol: Now that's how you know you own a Pontiac.:wink2:

Merry Christmas.


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## Chris-Austria (Dec 23, 2010)

That's a perfect description of how it has to be 
Thanks!


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## 666bbl (Apr 13, 2014)

1st, Merry Christmas to you and yours. 

That done, I'd like to add a little aesthetic note to piston slap when cold. When you hear it, when you know what it is and why, it should warm the soul just a wee bit. It means "...something serious in there..." and you know full well it's gone in a matter of a few minutes. The last car I had that had slap was a 95 T/A. Sure, it was a Pontiac if in name only (generic GM 'F' body) but that old LT1 had forged pistons. Yes, they slapped when it was cold. A reminder that when I did top it out to over 140 MPH (4 separate times!) I knew it could take it. I knew they spent a lot of money developing that engine to last under the foot of the enthusiast it was designed for. My race motor slapped pretty good. 1st fire on race day you could hear it over the open headers. It went away fairly quick but there was that little invisible grin inside. I knew it was normal, it reminded me of the love that thing got when assembled. Like solid lifters, clunking positraction on turns, aromas that only emanate from certain cars, just embrace it. 

Oh yeah, and happy new year too.


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## Chris-Austria (Dec 23, 2010)

Thanks 
I'm not sure if it will even have piston slap when cold, I cannot do much about clearance anyway (I can increase it but not decrease). I think I'll run it with the 0.0045 to 0.005" and that's just what Ross wants and Butler said it's okay.

At the moment I'm dealing with other clearance problems.. the Scorpion Race full rollers won't fit that easy... I'm not sure if the Endurance would fit but I decided to go with 3/8" spacers from Butler, they say they prefer the Race version and with the spacers they will fit.


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## extinctmake (Feb 8, 2011)

While hardly near a race type engine such as yours, the old 389 in my '65 GTO sounds like a diesel when it is cold. 

I thought my valve train was out of adjustment and my old friend who built the engine and used it in his '57 Pontiac came over and showed me how to set the valve train. The noise remained and he informed me that he built the engine to be a little loose and the sound was piston slap when the engine was cold. The noise goes away once the engine warms up. 

I have long range plans to rebuild the numbers matching 389 to my GTO. I really would rather not have the slapping noise when the engine is cold. In the meantime, I will live with the noise and not worry about it.


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