Hi there, I've got a ybr custom 125cc, love the bike just don't like the speed. I'm looking to get a big bore kit fitted, now are they easy to fit?? Do they make much difference??? Thanks in advance

Hi james , its 4 stroke , so its more complicated than a 2stroke , I.M.O. I would save the cash for your test, then upgrade, 300-600cc

Setting it up to TDC, if its a OHC (dobt it is) then you need to keep the cam it the correct possion, then take off the head, then remove the barrel, take out the gudion pin locking ring, then remove the gudion pin from the piston, fit new piston and rings (the correct way) fit new bigger gudion pin, fit new barrel without damaging the piston rings, re-fit head and cams etc.

Turn engine over by hand to make sure your not hitting the head with the piston. if all is ok then fire it up.

Setting it up to TDC, if its a OHC (dobt it is) then you need to keep the cam it the correct possion, then take off the head, then remove the barrel, take out the gudion pin locking ring, then remove the gudion pin from the piston, fit new piston and rings (the correct way) fit new bigger gudion pin, fit new barrel without damaging the piston rings, re-fit head and cams etc.

Turn engine over by hand to make sure your not hitting the head with the piston. if all is ok then fire it up.

what about the other bits?

like a rebore(if there's enough meat to bore), carb rejet (poss a new carb needed)

Yer thought id let someone else complicate things a bit further

If you've not done your test you probably can't ride it legally with a big bore kit... If you have done your test, buy a bigger cc bike, much easier, more reliable and the insurance companies won't rape you for modding the engine.