You take the head off and see what size piston is in it.
No number on top is stock (50mm).
Then you buy the next size piston and have the cylinder bored to the new piston size.
I can do this for you if need be.

 

Well finally got a change to tear the quad down. Piston wasted and the cylinder scratched up. Going to replace it all. I found a complete kit on Ebay for $150 cheaper then having it bored. Anyone know some where else to get the kit? also looking for the block off plate for the oil pump.

 

LT80 back in action. Replaced the whole top end, removed oil pump. Cooling fan blades broke replaced the fan also. Starts right up now. I am guessing there is a break-in period. So I have adjusted throttle back half way and going to run a tank of gas through it, before letting him go wide open.

 

I always use gasket silicone and a good ole shinny new quarter for the oil injection block off plate and re-use the 2 mounting screws to squeeze it down! Gives it that cool look!lol

 

I have the same problem, put my finger over the spark plug hole, felt compression so i then checked spark, had spark and put starting fluid in it and it didnt even spudder