Hi, thanks for the willingness to help fellow riders all!!!! I have a 2001 wr426f. Blew 4th gear while riding. I do mechanic by the way but, this problem has me stumped. I split the case and removed the wr gearing from the main and drive axles and installed 2002 yz gears, new shift forks, new shift stop and shift stop torsion spring. But, the tranny will only shift into 1st, 2nd and 3rd gears. wont even pretend to go into 4th. Any ideas???? From what i have herd, the gears should be interchangable. Anyone have any advise for this confused mechanic? I have split this case about 15 times now and cant figure it out. thanks all.