While non of the HRC manuals for the NSR say you can Honda often listed 18" rears for other bikes such as the RC30 and RC45 to go along with the more common racing size then of 17". So while there may be little benifit in doing it its not unheard of.

Of course HRC then intended you to alter the ride hight etc with their shock which unless you have a adjustable shock you can't do to compensate.

However a '21 with a little more rear ride hight isnt a bad thing as I'm sure Andy will agree too.

Give it a whirl. The later MC18's ran a 150 tyre the same as a '21.
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If I have to take the carbs off once more...