Motorcycle Repair: My 1996 Yamaha yz250

Question
I wanna bore out my 96 Yamaha yz250 what should I get it bored out to

Answer
Hello Lucas,

My advice to you is, do NOT overbore your bike.
The cylinder wall is hard coated aluminium (NiCaSil), which means that over-sizing is not so simple, since there is replating involved.

The gain in power from increased displacement, diminishes due to alteration of the ports timing/area/angle. Additionally, the YPVS valve limits the maximum diameter attainable, without extended modifications to the exhaust port.

If you need more power, have your cylinder and crankcase competition-ported and the entire engine blueprinted to factory specs. Install an aftermarket racing forged piston (Wiseco, Wossner, ProX) and fine tune carburation and exhaust.

If your issue is just a worn cylinder though, Wossner offers slightly oversize (68,01 & 68,02mm) piston kits, to compensate wear.

I hope I was helpful, but if you need more info, don't hesitate to ask.

Best Regards,
Lazaros.