Yamaha YBX125 bike Re-bore?

[balam]

my question if i rebore my bike to 0.5mm is that necessary i should replace my old piston,, i have a doubt whether a re bored egine must be replaced by a new piston ,, why is it so ?

 

[Steve Y]

0.5mm is quite a heavy rebore. You have to get an oversize piston otherwise it will slap around inside the cylinder too much. Also, it won't seal properly and you will lose compression, and air/fuel mixture will be forced into the crankcase, thus contaminating the oil, hence reducing its effectiveness to lubricate and possibly leading to catastrophic engine failure.