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Forged Piston Pb Clearance Variance


quick31
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Im currently in the process of building a rb30et with the works hoping for 350rwk, the only deli ma i have run into is piston selection not based on price but rather long term reliability. First of all, i don't want my engine to piston slap when cold so if i am going to opt for a forged piston it will be with the 4128 material to irradiated slap with correct clearances. Through past experiences which forged pistons have the tightest PB clearance i belive its cp's at 3.5thou also understand the more power you make the more thermal expansion and the higher chances of ring butting with tighter clearance's but i really want a nice tight motor any advice will be much appreciated.

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From memory my wisecos are set at two and half thou.

Fark thats pretty tight how much power are you macking and is any piston slap present. If im going the forged route it will have to be the wiseco pistons they are the only company that make their own forged blanks their quality control is second to none.

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Yeah I was going to go 2 thou but only went 2.5 because I'm thinking about putting some nos into it later down the track.

All the wisecos have teflon coated skirts now (I can think of a few girls who could make use of this) which takes up about 0.5 thou on top of the piston itself.

I do get a little slap on cold mornings but it doesn't last long.

Wiseco's have the longest skirts of any RB25 forgies that I know of though which means they will stay more square in the bore and probably keep the bore itself square for longer.

My engine had about 100,000 k's on it when I rebuilt it and the stock pistons had made the bores oval. I went to 87mm (40 thou over) because Wiseco (and most other manufactures) don't make 86.5 which sux because you can only bore it out twice before you need to sleeve it instead of 4 times when you have half sizes available.

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wiseco recommend 4thou on their forged 4128 pistons and 5 to 5 1/2 with the 2168s. What power level are your macking abo bob lol

5 and a half is massive. I don't know which ones I have. I am making about 260-270rwkw. 8.75:1 compression ratio, highflow turbo.

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Piston to bore clearance is relevant to piston metalurgy. But, as usual, you get nothing for nothing. More silicone in the alloy means tigher clearances can be used. But piston strength suffers. So a forged piston that will run at 2 thou won’t be strong as a piston that runs at 4 thou.

:P cheers :D

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Tomei have 86.5 mm pistons you can have 2"thou clearance but you must use a torque plate to bore it out and hone it. You won't have piston slap at all cold or hot .

What SK said is right though but for the amount of power you want they will be fine. I must say they are a little pricey though, like all jap pistons .

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