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Acceptable piston diameter variation in new build

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I bought some new shelf pistons from JE Pistons for a Subaru build and they sent me three right side pistons and one left side piston. JE Pistons have now sent out a replacement piston, but where the first three are sized 99.47mm, 99.48mm, 99.48mm (all from the same batch #), the replacement is 99.455mm and from a different batch, but otherwise visually identical.

The weight of the new piston is identical to the 99.47mm piston at 396.9 grams.

I haven't had the bores honed to suit yet, but assuming the final piston-wall clearance is consistent and the piston+pin+rod weight is balanced across all four pistons, should I be concerned?



That is a little surprising from a manufacturer like JE. Typically when I measure a set they're identical to less than a hundredth of a mm and I'd definitely be surprised to see a variation of 0.025 from smallest to biggest. Personally I'd be inclined to raise it with JE but of course if the block hasn't been machined as yet you can have the machinist tailor the bore size to suit each piston. Given that you're talking about 0.025 mm or 1 thousandth of an inch, you're going to need to ensure the correct piston goes in the correct location though or your piston to bore clearance is going to be royally messed up.

Thanks Andre. You're echoing my thoughts exactly. I've reached out to JE for their insight. The only shadow I've doubt from my measuring arises from trying to mic in the hole of the "Perfect Skirt" coating. If you haven't seen it, a small hole in the skirt coating is left bare for the purposes of locating the mic faces at the nominal piston diameter. The problem with it, is that the hole may be too small to fit normal mic faces on there. That said, I was able to repeat my results five times on each piston.

Yes we have just received a set of pistons with their perfect skirt coating and I agree, the hole in the film is too small for most mics to fit into. This certainly could be the cause of some of your issues but I'd still expect a consistent 'error' in your readings across all the pistons.

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