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Piston to wall clearance

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Piston to wall clearance

Postby JOHNZ » Sun Nov 07, 2004 3:11 pm

Hello everyone,

Using Wiseco pistons in a 15:1 555 BBC drag racing engine...manufacturer calls for .0052 piston to wall clearance..

I had my block honed recently and ended up with 4.562 bore...

seting an outside mic on the piston skirt where Wiseco says to measure....then zero a dial bore gauge to it and use it to measure the clearance in the bores.

I end up with .0085....is it safe to run that loose or what can I do to bring the clearance down to the .0052 or so area?

Thank you.

Postby Bumps440 » Sun Nov 07, 2004 3:52 pm

WEll I am far from an expert on this topic or any other topic compared to many on this board But, AS I see it , you could run it as it is with a little excessive P/W clearance or you could look into having the skirts coated, which will take up some of the clearance and also at the same time provide a low friction coating.
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Postby CNC BLOCKS » Sun Nov 07, 2004 4:35 pm

That seems like alot of clearance to me as far as Wiseco, JE, CP, Diamond pistons etc. the clearance is built into the skirts and I have found them to be with in .0005 of what is the required bore size. Having the skirts coated will help.
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Piston to wall clearance

Postby JOHNZ » Sun Nov 07, 2004 5:52 pm

Thanks guys,
the problem is that the pistons are made for the 4.560 bore and the machinist honed the bores to 4.562 it basically added another .002 to the clearance....

Not sure that coating can add so much to the piston diameter (maybe .001") so I am kind of stuck!!

I guess I could run them as is but I think that .0082 clearance might cause some issues.

Thanks again.

Postby speedtalk » Sun Nov 07, 2004 5:59 pm

I'd run them.
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piston to wall clearance

Postby JOHNZ » Mon Nov 08, 2004 7:47 am

Makes me nervous to run the clearance so wide, I just have not done it before...I will call Wiseco this morning to see what they suggest.

Don, great site by the way...first post for me, but have been reading for a couple of weeks.

Thanks for the suggestions.

Talked to Wiseco

Postby JOHNZ » Mon Nov 08, 2004 10:33 am

Talked to them this morning, they said that you can safely run them up to .0085...

I will try that and see how it goes..


Postby Torquemonster » Mon Nov 08, 2004 1:11 pm

I'd call Swain.... for 2 reasons - 1 - it'll slightly help reduce clearance and 2 - it will greatly help offset the increased propensity to rock


Postby Adger Smith » Tue Nov 09, 2004 10:36 pm

You failed to give me enough information about your engine to comment.
Heat the piston & block. How much did they grow, the same rate? Does the piston move faster when heated to the running water temp of the block. Is the block filled? Most clearances are considered minimum. I agree with the coating. It will also keep the cyl wall cleaner & the rings sealed better.
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More info

Postby JOHNZ » Tue Nov 09, 2004 11:49 pm

I am not sure what more info on the engine you would need....block is not filled, it is NOT an aluminum block, they are forged pistons...I have not heated them to see what the growth is, I should try that but a doubt a forged piston would grow that much...

Hope this helps. I have thought about coating, not sure I want to pay the $300 right now though.

Thanks for your suggestions.

Postby rfl28 » Wed Nov 10, 2004 4:45 pm

hey john... run them....loose is fast...wiseco is one of the top pistons made....i use them often w/ great success....their skirt design does'nt seem to collapse like other brands when put together w/ some extra clearance...good luck,..bob
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Thanks for the help

Postby JOHNZ » Wed Nov 10, 2004 4:55 pm

thanks Bob and everyone, I heated the pistons some and the clearance got tighter...so I am gonna run them and go from there.

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