Piston replacement

Ok guys, here’s a first for me - piston replacement. I have an IAME X30 125cc shifter. Technical document states bore when new is 53.89mm, so using my micrometer I zeroed my cylinder bore gauge to that. Measurement taken both directions, at top of cylinder (area above ports) was .04mm greater than this, so 53.93.

According to the “basic instructions” document for this engine (screenshot below), “the prescribed clearance between cylinder and piston, when the piston is new, is 0.10-0.11mm.” That would mean I need a piston that is 53.82-53.83mm. Right?

I go over to my main parts source - mondokart - and I see 53.83mm among my options. Screenshot:


Do I sound like I know what I’m talking about, should I pull the trigger? I find the best prices at mondo but returns aren’t really possible, so thanks all for any help giving me the confidence to make this purchase!

No no no

IAME piston sizes relate to the size of the bore, not the piston. If you measured your cylinder at 53.93, that’s what size you order. IAME have already accounted for the clearance.

Green dot is nominal
Red dot is 0.01 bigger for a tighter clearance, for tuning - usually used by serious crack heads who change pistons each race.


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But…….that should be the piston size when new, not the bore which should be 54.00.

Got it! Thanks so much, Richard. And non-Richard people as well, I can’t thank you enough for the opportunity to be able to ask questions. I try to figure things out as well as I can on my own, but I’m limited by my experience and this community is just so welcoming and helpful, with questions from basic to advanced. And my questions very much span that gamut. Thanks, y’all.

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