The MC18 I purchased was rebuilt with single ring pistons, and according to the mechanic (a pretty well known nsr expert), it's no good at all. So I got him to order a complete top end kit from japan. He said I was pretty lucky because he mentioned something about it should have damaged the barrels over time but it must have seized before any real damage was done. I'm not sure if that was due to the single ring, or possibly a poor rebuild or maybe shabby replating. Anyway he didn't recommend the single ring at all.