I just bought a blaster off of a freind of mine. Well it needs a new top end and when I bought it he gave me a jug for it, the jug looks ready for a new piston but it has a very small amount of surface rust on the cylinder wall. (It sat in his shed for two years). anyway can i sand it off with fine sandpaper or a dremel or something, or do i need a new bore.

 

u need a new bore for sure...using a dremal or sandpaper could..or should i say will end up costing u alot more money from all the damage it would cause...the cylinder walls have to be absolutely perfect...the slightest play in the piston will be costly...so do it right get it bored out and have a blast on ur blaster..hey u can tell all ur buddies u have a modded blaster if u do it...engine mods

 

The most important thing you can do is figure out why it blew up.I still have the first 4 pistons I replaced on my first cr 125,they are completely destroyed however the look exactly the same,It would run for exactly 7 days+blow up,back then I was just a parts changer.Now I know it was accecive piston to cylinder wall clearance.You may get lucky boring it to clear up that prob,but there are so many things that can make an engine go down I wouldn't begin to kid you that that is it.
Bad oil pump
water in fuel
sticking gas cap diaphram
lean jetting
air leaks
just a few possibilities I've seen