Flywheel explosion on 06

I want to see if anyone else has had this problem.

Last week I was out riding and the bike was working great. I parked the bike to help a guy who crashed and when I got back to my bike it took a few kicks to start. I eased away about 5 feet and it stalled. I went to restart and nothing. I kicked and kicked and nothing. So I pushed it to the truck. I checked for spark and nothing...., disconnected the kill switch and still no spark.

So I pulled the flywheel cover and found the problem. The flywheel was grinding on the stator. Everything was still tight but it looked like the lip on the insert in the flywheel (that covers the magnets) had lifted and hit the stator back plate. It peeled the lip off got inside the flywheel and ground the stator. I found the pieces in the cover and sump. I fished around in the sump with a magnet and I think I got all the bits out. Luckily I think the magnets in the flywheel collected the ground bits. The oil looked clean, I haven't pulled the filter yet.

The crank is tight up and down but moves slightly in and out. Maybe a couple of thousanths. Not enough to cause it to rub. everything turns easy with no binding.

So I went to the shop and ordered the flywheel and stator. They are checking to see if Yamaha will cover it(hopefully the parts cost $300). The stator was backordered and will be here the 15th.

I figure this is an isolated problem but you may want to pull the flywheel cover and check the flywheel to be sure the lip is not lifting. It is right on the face of the flywheel after you remove the cover. I would post pics but I left the parts with the shop.

The bike has 7 hours on it. Otherwise love the bike. I just can't wait to get it back running. Luckily I also have a KTM 525EXC so I can still ride.


First time i have herd of it, but Yamaha should pay for it, they payed for a whole new motor for my 250f when i blew it up.

I got my parts Flywheel and stator and installed them. Bike fired up and ran fine. Yamaha did not cover them but my shop gave me the parts at cost which was really cool.

So after I ran it I drained the oil to make sure there are no bits floating around. I pulled the access plug on the flywheel to see if I could see anything. I pushed on the end of the crank and it moved (not much, straight in) and made a slight clunk.

The movement concerns me. The shop says it shouldn't move. If you look at the way it goes together there is nothing stopping it from moving except that the crank presses into the left crank bearing. You have to push hard to get it to move but you can do it by hand. Maybe I'm paranoid???

So I pulled the clutch side case to make sure the crank gear on that side was not loose. It was tight. I put an indicator on the crank and made it move. It moves about .015" straight in and out no vertical play. The manual does not call out any tolerance.

Now I am nervous about running the bike. I don't want it to lock up over a big jump or in the whoops.

Does anyone know if this is normal? or can someone pull the stator cover, grab the flywheel and give it a good push and pull to see if they all do this?

It's an 06 450. I am also going to try this on a friends 250F, but I am not positive it would be the same.

Thanks in advance,


The small in and out movement is totally OK. That is in fact what makes a kart engine a winner when renovating the high revving small powerplants. I'm no engine builder but seen how they seek for that movement with a hammer when putting a crank into cases/bearings. Ask a reputated engine builder and he'll tell you the crank needs that small movement.

I would say that the movement is cool. Thats just the slack in the roller bearing. Ther has to be a small amount of slack for oil, when that thing pressures, i bet it wont move. My opinion, but all rollers move a tad in and out.

Check out my post about the 06 flaw? Tell me if your bike had the same symptoms leading up to it?

I checked a friend's 250F and it did the same thing and it was easier to move.

I thought about the bearing play but the shaft is moving in the bearing. There is no (or probably very little) movement of the inner race of the bearing.

Thanks for the reply's. I think it is ok since I found the same movement on another Yamaha.



I checked the 06 problem post. Mine may have sputtered a little but not too bad before I stopped and it wouldn't restart. I remember thinking I was running out of gas a lap or two before I stopped. I was on a MX track and was riding pretty hard so I was on the gas a lot more than I would have been trail riding. Also it was bone dry when I was riding.

I would pull the flywheel cover to make sure, it only takes 10 min. Lay the bike on its side so you don't lose any oil, pull the shifter (you may not have to but it makes it easier), and pull the cover. Check the lip around the face of the flywheel. There is a sheet metal insert that covers the magnets. Make sure it isn't coming out or rubbing on the stator. Also inspect the area where the coils align with the flywheel for wear.

And push and pull on the flywheel to see if it moves while you are in there. :thumbsup:

You will also be able to see if any water got in there. It is worth checking since once it goes the bike has no spark. Don't want you getting stranded out in the woods.

Good luck,


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