911...911... MAYDAY MAYDAY...EJECT... EJECT... EJECT... Oh No!

I am in complete shock and disbelief.....Right in the middle of the GNCC season I decided to upgrade the 10oz flywheel weight to a 12oz Flywheel weight.....

It appears that the individual that put the 10 oz flywheel on used an air wrench and severely over torqued the end of the threaded shaft. Not knowing this, I put my trusty 3/8ths socket and ratchet on the nut and began to loosen...on the second turn it snapped and off it came.




This is a crazy thought I know.....But what if I take the bike to my friendly master welder and have the new flywheel welded to the end of the shaft.....Will it work? I have ordered a new left side crank, bearing, full gasket set, and a new timing chain......But my local shop wants $575.00 for the labor and that is just not in the cards at the moment.

So what do you guy's think ? I could always race the WR for the rest of the season, but It's way too high compared to the YZ and much heavier.

Bonzai :)

Oh My Gad

That ain't right, That brought me to tears :)

Man tough call Kaze, honestly, What if it fails just worst case scenario

I would wait but thats me, I can afford a worst case scenario :D good luck

Buddy sorry to say but no matter what your screwed :cry:Pay-em big now or even bigger later. Yes I think you could put a couple of 1/4" spot welds. But it will become an almost permenant fixture on your bike, for future r/b and maintaince you would have to grind the welds off. A good tig/mig guy might be able to fab you a new threaded section and weld that back to the broken shaft and then clean up the threads. Not impossible to do but very labor intensive. Let us kow what you decide and how it goes. I can just imagine the sick feeling you must of had when SNAP! :D Good luck I hope this helps :)

Hey Yamakaze,

In my books you have enough thread for a nut to work. The problem is the flywheel face sticks out flush with the brake. If you can get the face of the flywheel machined with a countersink of slightly larger diameter than your socket you might have enough meat to lock the flywheel down to the tapered crank shaft. Not seeing the part I hope you can countersink 3/8" deep for the nut to grab the remaining threads fully. Forget welding it. It will brake since Yamaha makes these shafts just large enough to handle the stresses or hard riding. I hope this helps. Sorry to hear about the Monkey that did not use a torque wrench! :)

welding it will get you thru the season.have getting it off when its time to fix though.

I don't know what hardness the shaft is, but IF it is soft enough you could drill and tap for a bolt to hold it. Yes, the bolt would be a smaller diameter (use a high strength bolt) so you would need to do a "WR450" mod; that is, if necessary lap the flywheel to the shaft for optimum surface contact (or check it with prussian blue or equivalent to make sure its OK), then clean and glue the whole thing together with loctite and let set for 24 hrs or so. The bolt would be used to torque the flywheel/shaft together for proper taper fit - then afterwards it would act as a safety lock (it too should be loctited) this whole thing being along the lines of what the WR factory mod does. Since the shaft taper diameter increases toward engine center, you can go with a large enough diameter and long enough bolt (long for providing equivalent thread strength for your tap threads in the shaft) to obtain the proper torque value. Though the edge distance at the broken taper end would be minimal that area wouldn't do much holding anyway.

I think it would work even as a permanent fix if done carefully (again, hardness is an issue). I've managed to keep other machines going with similar mods. Good luck!

P.S. Be sure the end of the shaft is trimmed enough so the bolt-head/washer when fully torqued does not contact it.

I feel your pain! Drill, Tap and use a very hard bolt with a large washer and some good glue. You cant hurt the crank, welding might cost you a new flywheel. Crazy as it sounds it could run forever like this. I have had to do this on small engines with no problems, good luck on the repairs.

OHHHHH dude! :)

These guys seem to have a fix for ya. Best of luck.

Seek and destroy the geek with the impact wrech!!!! :D :D :D :D


If you like the chassis better on the YZF, put the WR motor in it, have fun and buy some time on your solution.

tc that's the best idea i've heard yet,swap motor's or just ride the wr.Trying to weld the shaft is a half ass fix that will just create another problem later.just my .02 cent's :)

You could also ask about trying to weld a piece of material back on and running a die down and cutting new threads on the shaft.

OMG What FOOL didn't use a torque wrench??? I'm about to wet my pants :D

You've got a few options: You can either jimmy rig it, ride the WR or swap motors.

If you want to jimmy rig it pull the good ol wire feed and weld it back to gether grind the bead down even with the metal and rethread it.

Me i would just ride the WR and just wait for the moo to get it fixed right. Or if your lucky you know someone with a metal lathe and you could have a new one made.

This is all i can say :D :D :)

The swap motors or drill and tap has the most merit. You gotta ask yourself where is the heat going to go if you weld. The heat treat will really be messed up at the minimum. Always gotta ask what are the consequences / cost of it failing and how much will it wreck if it lets go. The collateral damage could be a lot more for welding as the FOD potential goes up.

Just weld it. If you have to remove it later, a die grinder will do the job. A couple small tacks won't cause much heat if done carefully. I can't see it flying off so whats the worse that can happen? Just make sure you have the weight you want and enjoy. :)


Swap the motor for the WR. Do your self a favor and fix it the right way. This is part of your crank, something I would not do half assed. Especially if you spin that motor at 13,000 rpm.

Just my .02.

Goodluck with your decision!!


Yikes!!! Not being mechanical in any way shape or form, is there a kind or reverse stud that you might be able to screw onto the shaft? This stud would then have the same diameter threads extending out so you could use the new threads that you just screwed on????


Do you guys think if he just welded the shaft, it would cause an imbalance and spin a bearing. I like the engine swap idea. A temporary band-aid would probably cause more damage and money then it's worth.

Just my 2 cents

That is exactly why I mentioned swapping motors, verses welding or tapping the crank. Even though it is a motor cycle engine, that bottom end has to be well balanced to be able to spin at 13,000 and not spit parts every where.


A good friend and a Certified Yamaha Mechanic has picked up my bike and given me what I think is an extremely fair price to R&R the motor from the bike and have it race ready by this coming Sunday.

Parts: at Cost $235.00 (Left Side Crank/Nut/Bearing/Timing Chain/Complete Gasket set)

Labor: $350.00..

Total for Repair: $585.00

Which includes replacing the timing chain and checking the valves (shim as necessary)

Way Cool.....My local Shop was charging $575.00 just for the labor. My buddy does Great work and has always treated me fairly.....I'm thinking Bonus on this one.

Bonzai :)

thats super cheap!nice to have a good friend! :)

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