The idler gears are causing the woodruff key failures.

The reason the woodruff keys are breaking on the 03 WR450's is because the idler gears are constantly engaged to the flywheel and starter. They are not designed to spin backwards. So, when a kick back occurs, the flywheel can't turn backwards so the woodruff key snaps.

Yamaha of Japan has a new design for the idler gears that allow them to slip at a lower torque so that during a kickback condition the flywheel will not be mechanically locked on the starter.

Yamaha USA either does not know about it, or they are playing dumb. Check out the posts at for proof.

that was only part of the problem. they also didnt have the taper {crank to flywheel} right. but both problems have been changed for 2004. mine is on the way :D :D :)

Has anyone confirmed that the tapers are wrong?

I bet if anyone removed his idler gears from the front of the starter, they will not have another woodruff key failure.


It is not the gears at the front of the starter which resist the clockwise motion of the crankshft in the event of a backfire it is the starter clutch gear situated on the crankshaft directly behind the flywheel.

Regarding this backfire theory, when the woodruff key went on mine at 11km the bike backfired when it failed to start after a stall but IMHO this backfire occured post key shearing as the timing slipped. On strip down the sheared key was not a good fit in the flywheel or the crank.

I carried out the lapping, loctite mod and manufactured a better fitting key. Have done 130km since without any problems. Checked nut torque today - didn't budge.

Why no backfire to cause problem now?

KGD :)

Please explain this backfire that people are refering to. Mine isn't a loud pop like the timing is late or the gases are exploding in the exhaust pipe. Mine is just kind of a cough and I'm not even sure the ignition is fireing (fireing early) and trying to make the engine spin backward. If its the early ignition firing how is richining up the pilot circuit supposed to help this like some people (and Yamaha) say? My only guess is that the leaner mixture takes less compression to ignite and if it is actually igniteing wouldnt it be a louder pop. It really doesnt sound like any backfire I've heard before. You would be getting more noise or pop from the airbox wouldnt you. I've had a BRP (Honda XR650 catch fire from that kind of backfire). Is the automatic decompressor involved? Lets hear from some real experts please. Tim

This is a copy of a post on ""

I tested the torque slip and sticktion of gears by holding the larger gear

in a bench vice with a 2mm thick peace of copper rapped around it. I grabbed

the smaller gear the same way with some large set of hand held vice grips

and attached a set of fish scales 150mm from the center of the gears. I then

pulled on the fish scales and noted how many Kgs it took to move the gears

and calculated the Nm.

The first time I tested the old gears it took maybe 50Nm to move them.

Testing them several time straight after that repeated 40Nm. When I tested

the new gears they slipped every time at 27Nm. It seemed to me that the old

gears had probably never slipped since they were made as they a lot of

sticktion when I first moved them. With the new gears I marked them before I

installed them so I new they had slipped recently before I tested them.

I understand sticktion is an effect that happens with friction between

movement parts and is always there to some degree. It takes more force to

first overcome the friction than it takes to keep things moving. Also the

amount of sticktion varies a lot with materials and time between

movements..... but it sound like yours are seized together.


I thought I saw some post about new gears coming out for the start that have a disengage of some sort. Anyone have any info on this??

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