shifting problms

i have a 2006 yz450f and im having some shifting problems, 1st, 2nd, and 3rd all work fine untill i get into 4th and 5th rigth when i shift up it it seems like the gear only engauges half way then slips out and itll slip in and out untill i shift back into 3rd and it does it in both 4th and 5th gear. its kind of hard to explain but it sounds like there is alot of grinding going on. can someone tell me whats happening and what i can do to fix it?

This is a result of wear at the locking lugs that hold the 5th pinion to the third pinion so as to engage 5th gear Both gears called "pinions" because they are on the main shaft, and drive the associated "wheel" gears on the output). The lugs are originally very slightly undercut, or at the very least, dead square with the shaft axis, so that they hold together under a load. As they wear, the undercut becomes rounded and slightly reversed, so that the two gears repel each other under pressure instead. This forces the sliding gear of the pair away from the other one against the shift fork until they become disengaged. Once disengaged, the load is off, and the fork will force the two back together as soon as the lugs rotate into alignment. Then, of course, the load is restored, and they push back apart, repeating the cycle.

To repair this, in the case of 5th, you need, at minimum, the 3rd and 5th pinions and the #2 shift fork. Cases must, of course, be split.

Im not very engine technical so Ill show it to my buddy and see if he can fix it...thanks

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