03 yzf 450 tranny problems

has it been a problem that has plauqed these bikes 4th gear wont hold

Not the 450 very often, no. The 426 has had perhaps more trouble than it should have with 4th and 5th, but except for an occasional broken third, usually on a harsh landing, they don't have much transmission trouble at all that I know of.

its weird all the gears look fine shift forks are fine the only this we can think of is the shaft that 3rd and 4th gear sit on was faulty from the factory it happend a month after bike was purchased but yamaha would not cover it . it has sat since because no body can figure it out

You are fortunate in this case, because the main shaft is located along its axis by the bearing in the right crankcase half, same as the shift cam is. Since 4th gear is engaged by two gears on the main shaft, that makes it a much simpler thing to check 4th gear for proper engagement depth than if it were a pair on the output shaft.

Place the assembled main shaft, shift cam, and fork in place in the right case half and install the shift cam locator plate. Shift to 4th, seat the main shaft against its bearing, and check the mesh of the dogs.

Your problem could indeed be a defective shaft (although it's rather unlikely), but it could be any of a number of other things, too.

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