You never need oversize rings for normal maintenace. The cylinder is repaired if damaged, then replated and honed to the same specs as new, so the pistons are allways the same size.

One ring pistons are used in the RS250 and can be used in NSR with some disadvantages, that are probably described here on the board somewhere
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Poul
"If Life Gets Boring, Risk It!"
MC21SE, KISS box
MC21SP, HRC box, racing loom