Given that the "newest" NSR250 is now 17 years old, and consequently you cannot often be sure if some barrels have already been replated or not, I would advise you concentrate more on piston-to-bore clearance than actual sizes.

OEM clearance is 0.035~0.044mm, with a service limit of 0.08mm. In my opinion, 0.08mm is VERY slack!

Those numbers should give you an idea of what's sensible to run, however I would say that Matt does go to extremes with race motors, that aren't necessarily convenient or appropriate for the majority of owners, even those who trackday a lot. As an example, Stephen RC45's MC21F3, making in excess of 75hp, always ran the same clearances, regardless of track. Not saying he's wrong, far from it, but he was racing against a Honda/HRC backed team when he was building those motors.
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Andy.
NSR-WORLD.COM

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