Newbie from Northern Utah. Piston size vs. Cylinder size

[jaredc00]

Hey, thanks for all the great info on this site. I've used it for a while without being a member. I ride up in Northern utah. Got a ATC250SX and YTM200N.

Now I have a question so I signed up for membership. Hope I can contribute as time goes along, but I'm just not that smart so we'll see.

Question about 85' ATC250SX: The cylinder measures 2.917" (74.09mm) the piston measures 2.897" (73.58mm). Looks like it should be stock 74mm cylinder with 73.90mm piston. Is that true? Do I need to buy a new piston? Walls look good, cylinder is round, should I buy a .010 over piston or stock size (if any)? If I do need to buy a piston and that piston needs to be .010 over do I take it to a machine shop to match the cylinder to the piston? Can I hone out the difference in the cylinder without taking it to the machine shop?

Thanks to anyone who might be able to help me out here. Thanks!

[adcotton]

I would take the parts and specs to a reputable machine shop and let them check it out. They need to measure for bore taper and out of round not just the diameter on one end or the other.

[WilliamJ]

The advice is good.
It needs specialist equipment to accurately measure a bore - the piston is easy enough to check if you know where to measure. The specs will state a maximum of 0.004" clearance at the widest part of the piston skirt (there is no point in measuring the top). The maximum bore taper and ovality will also be about 0.004" and the bore needs to be checked at three heights front to rear and left to right. It is just possible for someone who has measured lots of engines to check this with a feeler gauge, but not for a novice.
Bill