How to tell if I have a Big Bore Kit?

I hope this is in the right spot and hope this doesn't sound too stupid. I have a 2004 Rm-z 250 that is not running. I tore it down and found the piston ring was out of spec. So I'm in search of a new top end kit. Problem is, the previous owner said something about a big bore kit but didn't look too far into it and when I asked again they didn't know which one. Now what can I measure and what markings can I look at to see if I have one. 

Top of the piston should have the info of who made that, if its someone like Athena etc its probably an Athena BB. 

Take your cylinder to a qualified machine shop to have the bore checked. They will tell you what size piston you need.

Lol, nope. That thing is a POS and almost a full mm off in many instances. I see what you're getting at though.

OP, get yourself a good set of calipers as suggested. You'll use them more often than you think and they're great for making sure things are in spec.

Edited by 40acres

Excuse me, never ever take the initiative to do any work yourself. Find someone who will make you wait and charge you to do a 30 second task you could have done yourself. And heaven forbid buying anything other than the most expensive tool made. Some guys kill me, been doing this 20 years (cars, karts, bikes) and seem to be doing just fine.

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