Fs 650

the 2001 has a different waterpump shaft and also so is the clutch basket, the primary gear and the intermediate gear....the one above the primary gear. finally, the freewheel gear which we also call the sprague inner gear is also just for 2001. these must all stay together.

a 2002-2008 has a different cut to each and all the gears and the water pump shaft has a a drop from 25mm to 24mm giving it a shoulder for the correct gear to sit on.

you cannot mix these two generations. if you mix them you get a 'wh-i-i-i-i-r-r-r-r-r' noise.

probably
the 2001 gears are the correct classic engineering cut. unfortunately they make too much noise. the 'whir' is not bad if all the correct 2001 parts are used.

for 2002 the gears were 'undercut'. this is done when the gears are all soft still. then they go off to be hardened.

the 2001 parts had the 2001 part numbers printed on them so i think that they knew something was up, that a year later they could get it all sorted out.

Taffy