Sorry for asking, sounds silly but are the barells interchangeable through all NSR models?? I'm looking for some now, and I contacted a seller that claims he uses occasionaly "hexagonals" on his MC 21!!!

Yes you can use the MC18 barrels on a 21. The cooling isnt quite as good but myself and andy I think have now decided that the 18 porting is a little better than the 21's after looking at the port maps of each.
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If I have to take the carbs off once more...