Just a little FYI, the WR426 5th gear set fits perfectly onto the YZ426 transmission shafts. No issues at all. That being said, any and all gears between the WR and YZ will swap out, but you must replace them as a set. Buy both of the gears as listed on the parts fiche, and you will have no issues. The only substantial difference between the two transmissions is the main shaft (on the clutch) has a different first drive gear machined into each shaft, so if you like the tall YZ first gear, but want the rest of the WR gears, then just buy 2nd through 5th gears. If you want the short WR first but like the rest of the YZ gears, then replace the shaft and first gear wheel and keep the 2nd through fifth YZ gears. Mix and match at will, except for first gear. It sounds confusing, but it really is pretty simple.