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Matching cylinder to piston

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Hey guys, i have a member who i sending me his used cylinder which is in good shape, but im not sure what piston i need to order?

 

He said he measured the bore in several spots and got 53.98mm

So do i just order a stock 54mm?

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i also see lots forsale in good shape measuring 54.08mm etc. Any insight here?

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Hey There. What make and model bike are you working on? 

General rule of thumb is 1 thou per inch of bore size but this also depends if the piston is forged and if you have an exhaust bridge. I will talk more on this if you can tell me the Make and model.

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Hey Geof,

It's a 1998 yz 125. The currently cylinder is stock bore but scored bad. So I actually already ordered a jug with a 1mm overbore. Currently reads 55.08mm. Comes with a piston and ring to fit. But I think I wants to replace the piston and ring.

I found some wiseco kits for about 100$ at 55mm. Should be fine eh?

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Hey there. If I understand this correctly.......You are getting a new cylinder barrel that is 1 mm over size and they are also supplying a piston for this oversize. If this is the case the the piston will have the clearance already. If the barrel is 55.08mm the the clearance for the piston should be 2.16 thou or 0.054 mm......so the piston should be 55.026 or very close. It pays to check if you have a set of inside mics and outsid mics.If not take it to a machine shop and get them to check. You are looking for 2 - 2.5 thou clearance. Don't forget to endgap the ring and install it the correct way with the markings at the top. This is a forged piston and expands faster that a cast piston an for that reason I like to allow 3 thou. But thats just a personal preference.

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