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Old 11-20-2006, 07:53 AM   #1
Zerodog OP
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Location: SLC, UT
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Pissed Countershaft sprocket fell off yesterday on my 640.

On a ride out to a desert play area yesterday going about 75mph I pulled off an exit. Right as I rolled off the gas, CLUNK BANG, skidding rear tire for a second, then just coasting. I pulled to the side of the road and there was oil everywhere. I figured fuck, the engine case is broken. I might as well push this piece of shit into the great salt lake and be done with it.

Anyway I still am up in the air about what went wrong. The bolt was broken off inside the shaft. So it didn't just fall out. Or did it loosen enough to let the sprocket off the shaft then break off. I do check my sprocket from time to time and I have marks on the bolt/ washer and the sprocket to check it by just looking too.

Could the bolt have just failed from repeated overtorquing over the life of the bike? Or could it have just backed out even though it was loctited and checked once in a while? The bolt piece was still tight from loctite when I removed it with an extractor. And my reverse drills didn't make it move even though it grabbed. The piece left in the shaft was a little over 1/2 inch long.

I got the bike home and started looking it over. It was leaking so bad because the shaft spacer and oring came out. They were laying in my skidplate. I put them in and held them in place with my fingers. NO leaks.
The case looks ok. The main damage was the chain jamed in the frame and made a few little cuts and the lower guide got bent up. Luckily the chain popped off the rear sprocket and let everything freewheel instead of getting completely jamed in the frame and locked solid. The only question now is if the transmission is OK. It feels ok when I spin it in neutral. But who knows until I ride it again.

So the moral of the story is this. Don't just check those damn bolts, replace them once a season. I am ordering several sets of them now to just have them in the garage. They are really cheap....cheaper than a new engine.
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Old 11-20-2006, 08:06 AM   #2
Django Loco
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My 2001 Duke ll did something similar. The nut came off (they are supposedly
loc-tit'd at the KTM factory!) but in my case the little plastic countershaft
cover held the sprocket and nut on even though they were spinning free, the sprocket moved over enough to spin so I had no drive. But nothing got tangled up. Slipped back on and tightened it best I could by the side of the
road. It was loose again by the time I did 30 miles and home.

My dealer says this is, unfortuneatley, fairly common. He reccomended a
new nut and washer and RED Loctite. I did this, never had it happen again.
Here on the forums and elesewhere I've also read many reports of this same
thing.

Glad you didn't bust open the engine case!

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Old 11-20-2006, 09:37 PM   #3
meat popsicle
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I am told the CS bolt and washer are a consumable item - meaning they should be replaced every time you replace the front sprocket.

For folks who swap out front sprockes regularly, changing tooth counts depending upon the focus of the ride, I suppose they should change the nut and washer regularly but how often is a question best asked of the professionals (folks who talk about "bolt stretch" and metal fatigue...).

BTW, the bolt and washer share a part number and they come pre-treated with loctite. Of course if you swap tooth counts you should clean and reapply the correct loctite.

zerodog, I feel stoopid asking since you are an engineer/machinist, but you did use a torque wrench on that bugger eh? No over-torquing...
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Old 11-20-2006, 11:32 PM   #4
bmwktmbill
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Hey Z dog,
Good news that everything looks good and went back together.
Meat is right about the torque idea but no way could I lock the sprocket by myself and use a torque wrench. Not even sure I would have trusted a torque wrench for that application.
I replaced the bolt(pre loctited) and washer and used the 3/8 drive airwrench and just let it hammer.

I wanted the bolt tight after all I read. I don't see how you could break that bolt unless it was defective. To me the whole sprocket mounting system is well built.
Bill in Tomahawk, WI.
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Old 11-21-2006, 06:34 AM   #5
Zerodog OP
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Meat, I actually didn't use a torque wrench for this. I wish I did but.... As with most stuff I am finding on my bike that bolt has probably never been replaced. So it is my fault for not changing it when I did my sprockets and chain. After looking at it some more I am pretty sure the bolt broke off. Drilling it out it seems pretty soft. It most likely wasn't a grade 8 or 12 bolt. I don't really like the spring washer deal. I think it might be better to have a real lock washer type assembly with a highgrade bolt. I will take some pics of what I come up with.
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Old 11-21-2006, 06:44 AM   #6
Monkey_Boy
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Even a high grade bolt will fail if it becomes loose. So that is the key, bolt in proper condition, loctite, so it stays tight.

Can it not be wired?
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Old 11-21-2006, 08:32 AM   #7
meat popsicle
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Quote:
Originally Posted by bmwktmbill
...
Meat is right about the torque idea but no way could I lock the sprocket by myself and use a torque wrench. Not even sure I would have trusted a torque wrench for that application.
I replaced the bolt(pre loctited) and washer and used the 3/8 drive airwrench and just let it hammer.
....
I can do it by myself

You simply leave tightening the CS bolt for last, use your foot to depress the rear brake, which will hold everything nice and still while you torque the bugger down to the correct setting. This is also how I get the mutha off, but doing the CS bolt first. All this and more in my thread on changing the chain and sprockets.

I would guess, with some certainty that an air hammer, left to its own devices, could easily stretch the bolt and make it prone to failure. Bad idea Bill.

Quote:
Originally Posted by Zerodog
I don't really like the spring washer deal. I think it might be better to have a real lock washer type assembly with a highgrade bolt. I will take some pics of what I come up with.
The spring washer is NOT optional! The spring washer applies the correct pressure to the seal behind the CS sprocket, which keeps oil inside your engine... plenty of threads from folks who get a leaky countershaft, and it is finally traced to a weak spring washer or an aftermarket CS sprocket that is too thin.

Regarding the bolt: I guess a high grade bolt would be fine, but I always wonder if the designers specified a lower grade bolt so that it would fail prior to damaging what it fastens
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Old 11-21-2006, 08:41 AM   #8
MotoMike
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BTDT! Z-Dog, I have a tool for drilling out the bolt if you haven't already done it. It yours if you want it.

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