All normal NSR pistons have 2 different rings, the pistons/rings from a 1988 NSR250 mc18 and a 1995 NSR250 mc28 are totally compatible.

I'm unsure of the prices and availabilty in the states but looking at prices in the UK from Honda it's far cheaper to order pistons and rings from Tyga even if you use UPS for delivery, and you'll get the right bits;

2 x pistons and rings, UPS'd from Tyga £113
2 x pistons and rings, from David Silver £207
2 x pistons and rings, from CMSNL £220
Honda UK are even more expensive.
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