I've just bought one of these off the bay so will probably be needing pistons and rings at some point in the future. How different is the NS125R from the same period ('86-'87) as it has the same bore and the pistons are available in a few different sizes over bored, just the position of the gudgeon pin would be a worry as I've not got the NS250 so have yet to take it apart to look. But you can get rods the aswell as the stroke is the either same or close enough, skimming the castings or thicker/extra gaskets would sort that out, or use other rods with a suitable offset between the eyes.

If Honda saved themselves time by basing the 250 motor on the RS then whats the betting they saved even more by basing the 125 motor on the 250? Can't see if anyone has tried it anywhere on the web and it would save farting about with NS400 pistons.