06 YZ450F Broken Shift Fork Questions

[Flyin Ryan]

I have a 2006 YZ450F that broke the shaft part of the 2nd gear shift fork, this is the second time. I split the cases and the only thing that looked bad was the the shift fork, the gears looked good etc etc. I replaced the shift fork, bench tested it and everything seemed good........but 3 rides later the excat same thing happened again. Once again I have the cases split and looks like the only problem is the brokes shift fork shaft....anybody see anything like this before?? any suggestions??

[grayracer513]

Never heard of it. Where exactly along the shaft did it break? Under what circumstance? Anything unusual happen to the bike before it broke the first time?

[Flyin Ryan]

It broke both times where the roller pin that goes through the middle of shaft is that holds it to the shift fork. It is on my kids bike and the first time the shift shaft was also bent.....so maybe he hit something? But of course I replaced the shift shaft the first time and it is fine now.

[grayracer513]

Do you recall which way it was bent?

The only thing that comes to mind is that the output shaft could be bent, causing the gear to run off center, and that that may be applying a repetitious lateral force on the shaft. That seems like somewhat of a stretch, though.

Otherwise, all I can think of is that your son is a gorilla with size 18 shoes, and is stomping the living crap out of things going to second.

Are there any unusual wear marks on the fork?