Soon expect renewed cylinder. It will be made in zero tolerance, thats 92.00. Am in doubt as to use original or pro-x piston. By manual, piston diameter should be between 91.945 - 91.60. Probably new yamaha piston varies in this tolerance. In prox catalogue, there are 3 sizes, A, B, C, (91.92, 91.93, 91.94). So the biggest pro-x oversize is still smaller than minimum service limit diameter given in manual. If I check back, 1998 has smaller piston size service limit, that's 91.92-91.935. I presume they are offering '98 model tolerance for all 400's. Piston I took out, was running more than 3 years, closer to 5, was measured 91.95. Those are the only brands, i can lay my hands on with decent rebate, so no other piston brand is an option. Any thougts? Contacting prox is as talking to wall.