Andy wrote: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.
Fair call Andy. I see the most value for me in finding a nice compromise that doesn't need to be fiddled with all the time.
I guess the issue is determining which piston to order. It sounds like your advice would be to order the correct piston for the barrel? Followed by a careful tune obviously.
That being said, can you clarify if the C barrel is smaller or larger than the A and B barrels? It would be a shame if my spare piston was of no use (I will obviously measure it and check the clearance anyway, if I have the tool for it).