Well, after much contemplation, I decided I needed a wider 4/5 gear ratio on my 2010 YZ 450. I compared a few notes and part #s, and it looked like the gearsets from a 2007 WR 450 should drop right in, almost.....All the gears slipped right on except for 5th gear whl, apparently Yamaha used a coarser spline on that gear on the '10 models. Time for more thinkin' and scratchin'..Looked like if I used a 2007 WR output shaft, it should still be a drop in modification. (this output shaft is also the same one used on the pre '10 YZ models as well) I ordered one in and yup, it all bolted together and bench tested just fine.... Below are the parts I swapped in: 5TJ-17241-20-00 (4th whl) 5TJ-17141-20-00 (4th pinion) 5TJ-17251-20-00 (5th whl) 5TJ-17151-20-00 (5th pinion) 2S2-17402-00-00 (output shaft) 90209-21332-00 (washer) The above parts retailed for 275$ (+gaskets) but I shopped the internet and picked them up for 190$ or so. I'm recovering from a bit of surgery, and won't actually get out for a couple weeks yet, but it looks like I have a 4th gear that is 11% taller than stock, and a 5th gear that is 17% taller than stock....While keeping the stock close ratio 1-2-3 for squirting out of the corners...!!