Trans Problem Cases opened... now what

6 replies to this topic
  • 450GN

Posted June 26, 2007 - 07:42 PM


2004 YZ450F

Well this isnt my bike but my good friend and riding partner

well he has no fourth gear... everything else is fine except that he goes to put it in 4th and it just pops out... like it will catch for a split second... pop out... get in gear for a second... pop out etc

well we checked valves... all good
checked and replaced clutch packs... all good

so we split the cases and everything i see looks good so far

-now the left shift fork appears to have about 1/16th" wear on the bottom side of it...

-the "SHIFT DRUM??" well you can see where the right shift fork is supposed to slide down to the side for the 4th gear... but it hits on the corner slightly and wont LOCK into gear... COULD THIS BE THE PROBLEM?

-all the drive gears look great and show no major wear what so ever

i was expecting something major wrong and dissappointed i havent found anything i am leaning towards the SHIFT DRUM? but dont know... you can see a small amount of contact between the right shift fork and the drum and it is very difficult to rotate the drum and lock the right shift fork into place... almost impossible...

just what would cause this... i think this is the problem but could it really only be a faulty drum>

  • grayracer513

Posted June 26, 2007 - 07:58 PM


The only fault there would be with the shift cam (drum) is if the slot were worn wide at the point where 4th should be engaged. Very unlikely.

To cure this problem, replace the fourth and third gear pinions (on the mainshaft) and the associated shift fork (fork 2). The wear is not on the teeth of the gears, but on the locking lugs on their sides, which are what engages and holds the gears "in gear". The are supposed to be slightly undercut so they will hold in under pressure, but as they wear, they lose this shape and become slightly rounded. At that point, they start forcing themselves out of engagement under pressure. That bends the shift fork, which aggravates the problem. The bent fork will show up as a wear mark on the side of the fork between the two ends of the fork.

  • 450GN

Posted June 27, 2007 - 12:58 PM


thank you for the reply... .
would i also have to replace the 3rd and 4th gears?

and also i do not know the inside of the trans very well... just the left fork marked with L and the right fork which is R and is lower on the shift drum...

so the mainshaft is the shaft associated with the R shift fork id assume?

i dunno it is hard to imagine without it in front of me...

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  • grayracer513

Posted June 27, 2007 - 01:24 PM


It will not be necessary to replace the 4th and 3rd "wheel" gears (on the output shaft) unless they show some wear or damage that would indicate that they should be replaced independently from the problem you posted about. Fork 2 is the right side fork, and moves the third pinion back and forth on the mainshaft between 2nd and 4th.

  • 450GN

Posted June 27, 2007 - 02:24 PM


thank you for the help! graeatly appreciated

well we will order that one today and throw it together when the parts come in and hopefully problem solved...

thanks again for your time!


Posted July 16, 2007 - 05:02 AM


hey Im having the same problem. I put the bike in 4th and as soon as i give it some gas it pops in and out of gear.

What did you do to rectify the problem.

All my gears look perfect. Im thikning possible bent shift fork.

  • grayracer513

Posted July 16, 2007 - 08:38 AM


hey Im having the same problem. I put the bike in 4th and as soon as i give it some gas it pops in and out of gear.

What did you do to rectify the problem.

All my gears look perfect. Im thikning possible bent shift fork.

Trust me, they're not perfect. The wear/damage is not done to the teeth of the gear, but to the locking lugs on the sides of them. If you have a 450 4 speed trans that skips out of 4th gear under a load, you will need to replace the 3rd and 4th pinions (on the main shaft) and the fork. The gears are the problem, and the fork is bent because of it. If you just replace the fork, you'll bend the new one right away, and the problem will return.

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