you really need to have a search around as this has been asked befor, but i beleive that 18 barrels will fit a 16 as will 18 carbs but i think thats about it.

in fact it was you : http://www.nsr250.net/forums/viewtopic.php?t=2389
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MC21SP - Roth-tax
CBR400 - track
MC21R - sold
MC21SP - sold