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Motorcycle Repair/My 1996 Yamaha yz250

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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.  

Motorcycle Repair

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Lazaros Ampatzidis

Expertise

I can help you on EVERY issue involving POST-1980 Japanese Motorcycles plus KTM and Husaberg. My field of expertise includes Engine, Transmission, Fuel system (carburation & EFI),Electrics and Electronics. Maintenance, repair and tuning. Road and off-road suspension tuning. Please don't ask me about non-Japanese Motorcycle, especially American built on which I have low to zero experience.

Experience

I'm a rider and hobbyist since childhood. 25 years motorcycle workshop mechanic.15 years motorcycle racing mechanic. In order of experience high to low:Kawasaki,Suzuki,Honda,Yamaha,KTM,Husaberg.

Education/Credentials
Hellenic (Greek) Technical School.Internal Combustion Engines Engineer. Automotive Electronics.

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