I've been thinking it time for a top end rebuild on my 28 with 16000km on it. I guess if there left to long the skirt fractures and falls off the piston.

Whats the recommended mileage for a rebuild and where is the A,B or C mark on the barrel hidden.

the top end on an NSR is 20K and the bottom end is 30K.

The cylinder designator mark is generally found on the skirt area where the cylinder bolts to the cases (see the notes in the parts manual for your model for more details of this).
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Charles Gallant