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03 yz450f transmission fix

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I picked up the bike like two weeks ago and I knew it needed the carb cleaned but afterwards I found out fourth gear is gone. It is to the point where even cruising it pops in and out ever 4-5 seconds. So im gonna replace it. What is the fix? I thought about doing a wr transmission but I don't have the cash for the parts and can't find a complete used transmission. So what all am I gonna need to fix the transmission? It is just the fourth gear problem. All the other gears are good. Thanks

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Assuming that's the only thing wrong, you'll need the third and fourth "pinion" gears (3rd & 4th on the main shaft, or "drive axle") and the #2 shift fork (the forward one).

Inspect the bearings, especially the left side main shaft bearing, for roughness, and replace any snap rings you remove from the shaft.

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Assuming that's the only thing wrong, you'll need the third and fourth "pinion" gears (3rd & 4th on the main shaft, or "drive axle") and the #2 shift fork (the forward one).
Inspect the bearings, especially the left side main shaft bearing, for roughness, and replace any snap rings you remove from the shaft.


From the looks, I would say only the #2 shift fork needs replaced. The only wear I see other then that is the bearing for 3rd and 4th is a little rough but in gonna do it right and replace it. The fourth gear side of the pinion has an every so slight bit of wear on the corners. However, is the pinion on the shift fork a mated surface? So I would have to replace both.

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The fork is definitely bad, but there is "some" wear on the drive side of the 3rd and 4th dogs.  Doesn't look like much, but it's there.  It's your call.


Im probably just gonna be doing trail riding with this bike and fourth is gonna be few and far between. Would you replace it if you were doing it? To me it seems minimal and not much of a problem but in not an expert

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What I would likely do is to mock up the main shaft, shift drum, and #2 fork into the right side case half with the shift drum retainer in place and the indexing arm ("stopper lever") and engage 4th to observe how much depth of mesh the two gears have with one another.  If they overlap well beyond the wear zone, I would think they'd be OK.

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What I would likely do is to mock up the main shaft, shift drum, and #2 fork into the right side case half with the shift drum retainer in place and the indexing arm ("stopper lever") and engage 4th to observe how much depth of mesh the two gears have with one another.  If they overlap well beyond the wear zone, I would think they'd be OK.


I was working on putting it back together today but I didn't get far. I opened my new shift fork and I have the bigger one. The 5tb-6 or the one with the r. Now I am sure I order the number 2 shift fork, I checked back at my order and checked the numbers and stuff. But the 5tb-4 look like to me the one that needs replaced? At this point it's too late to return the one but did I get the wrong part? It just seems like im replacing the less worn part here.

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The shift fork in the front, the one that moves the 3rd pinion to shift 2nd and 4th is referred to as Shift Fork 2, and is 5TA-18502-00-00.  It's marked with an "L" because it tracks in the LEFT groove of the shift cam.  This can be confusing because some will view the rear fork as being the #2, but Yamaha orders things from left to right rather than back to front in thisn case.  Shift Fork 2 is their nomenclature.  If the new one is marked "R", it's Shift Fork 1.

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