Countershaft sprocket fell off yesterday on my 640.

Discussion in 'Thumpers' started by Zerodog, Nov 20, 2006.

  1. Zerodog

    Zerodog Long timer

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    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.
    #1
  2. Django Loco

    Django Loco Banned

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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!

    :deal
    #2
  3. meat popsicle

    meat popsicle Ignostic

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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...
    #3
  4. bmwktmbill

    bmwktmbill Traveler

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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.
    #4
  5. Zerodog

    Zerodog Long timer

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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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  6. Monkey_Boy

    Monkey_Boy Los bitchos atacan

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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?
    #6
  7. KenR

    KenR Long timer

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    What happened to you has been a fear of mine since I bought my 640. I just bought a new chain, sprockets, six new sprocket bolts and a c/s sprocket bolt and washer but haven't put them on yet.

    Going out to the garage right now...


    (glad your cases are intact!)
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  8. Hipster

    Hipster Long timer

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    One more thing, make sure the Loctite (liquid form) is fully cured before use.

    Tomc
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  9. meat popsicle

    meat popsicle Ignostic

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    I can do it by myself :wink:

    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.

    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 :uhoh
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  10. MotoMike

    MotoMike Washed Up Desert Racer

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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.

    [​IMG]
    #10
  11. PackMule

    PackMule love what you do Super Moderator

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    Glad you're okay, Z-dog. :knary
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  12. Zerodog

    Zerodog Long timer

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    Motomike I got the screw out ok with some extractors. Nice tool you made there! What I am thinking is machining a large washer with a clearanced area for the shaft on the backside. This way a thin after market sprocket will fit fine and you still compress the oring making a good seal on the inside. I can then use a star type lock washer that really digs in. And a longer grade 8 or 12 bolt. Longer isn't really any stronger but it does give more area for loctite. This way you get the torque needed for the o-ring and the security of the lock washer. The oring's compression is controlled by a step in the bushing. It can only get squished so far before the bushing bottoms out. This happens long before you flatten out the spring washer....unless it is installed backwards or it is completly worn out.
    #12
  13. meat popsicle

    meat popsicle Ignostic

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    Sounds pretty custom - I hope it works out for ya. Say, can you think of a reason why KTM didn't do it your way? (I can't, but then again I am not an engineer or a machinist.)
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  14. bmwktmbill

    bmwktmbill Traveler

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    Meat,
    The idea is to stretch the bolt, locking it. That's why you only use it only once and why the air wrench works so well is cause it stretches the bolt better than the torque wrench and assures you that the bolt is absolutely tight. This is also why it can come loose if you reuse the bolt.

    I get it that this isn't the KTM way but I have been changing countershafts since before there was KTM(on my Penton).

    In the old days we used a hammer driver for the same reason with good results.

    We were racing and to poor to buy new hardware. Or own an airwrench.
    Someone else has been down this road?
    We didn't have a torque wrench either but managed to run 6 -8 enduros and who knows how many MX's a year...for so many years.
    We always finished, just not first.
    Bill in Tomahawk, WI
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  15. meat popsicle

    meat popsicle Ignostic

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    Bill,

    I meant stretch the bolt too much. The torque specs are designed to adequately lock the bolt given the application without excessively stretching the fastener so that it becomes prone to failure.

    OH MACK! WHEREFORE ART THOU!?! :cry

    Zerodog,

    I recall the countershaft has some radial (?) play, if I used that wrong it slides in and out a tad. Perhaps the spring washer continues to allow it to do that while still applying load upon the seal... get it? What happens when you essentially stop the CS from moving back and forth? :dunno
    #15
  16. KenR

    KenR Long timer

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    If the spring washer is fully compressed by the c/s bolt, how could it allow any movement. It would seem to me that the spring washer is meant to be a locking device for the bolt, (along with some Locktite 243 of course) always applying pressure against the underside of the head.

    A number of years ago I attended a seminar by the fastener company Boman. Learned more than I can remember now about the importance of applying proper torque on a bolted connection. One thing that I've carried with me over the years is to use a torque wrench on critical assemblies. The engineers actually calculate how much stretch a bolt should have to stay in its hole and not shear off. I've used an impact wrench on things like this when a torque wrench wasn't available but it always made me nervous.

    Good discussion!
    #16
  17. Zerodog

    Zerodog Long timer

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    It isn't the sprocket that has the radial play it is the shaft itself.

    Ken, I think you are right about the spring washer. It is the locking washer for the screw. But there is for sure an issue with this design. I have read about lots of guys loosing sprockets and causing some kind of damage. There are also the 2 versions of the mounting. One, with some kind of nut and the other with the screw/ washer. KTM has changed this before because of problems. Manufacturers don't always do everything the best way. This is why the aftermarket parts companies exist. If everything was perfect from the factory why change anything at all?
    #17
  18. KenR

    KenR Long timer

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    That's certainly true, otherwise there'd be no product development either.

    I'm not suggesting that it's a perfect design, but I wonder if the problems folks are having with the c/s sprocket bolts breaking or backing out aren't due to a) re-use of the bolt and washer when they should be replaced at each sprocket change, b) failure to clean the female threads of the countershaft before assembly, c) not using a threadlocker or using the wrong type for the application (the new bolts come with threadlock already applied), d) incorrect torque being applied to the bolt, or any of the above in combination.

    I've never had the problem on any of my bikes with this design - the RFS's have the same way of mounting the c/s sprocket - but I'm probably just lucky! Glad yours didn't do more damage!!
    #18
  19. meat popsicle

    meat popsicle Ignostic

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    I am going to be obstinate here; I think you and wrong on this. Why use a "spring" washer at all? Sure the spring action of the washer could help lock the bolt, BUT - as you have pointed out - there are better ways to lock a fastener.

    Your logic might be OK if there is no intended purpose in the radial (or is it lateral?) play of the countershaft. Maybe its important that the shaft is allowed to move back and forth :wink: I am just trying to help here based upon what I think is right: if you lock that countershaft from moving and that movement is BY DESIGN then... ?

    I have not read about lots of guys losing sprockets, but when they do, yep, they can cause damage.

    I wouldn't be so quick to assume it was changed because of a problem. They may have found a way to improve the design. Remember these bikes are knock-offs of race bikes that are generally intolerant of problems.

    But once again: if you can come up with a better system, that does not have unintended consequences, I'd be all :ear

    Good points Ken.
    #19
  20. DustMeOff

    DustMeOff back on the 2 wheel wagon

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    These ramped lock washers are designed specifically for this type of application. They're a bit of a save your ass part if you have high vibration bolted connections, and can't keep bolts tight. we used them on hydraulic motor sprocket retaining bolts and they work wonders. its just a matter of finding one the right size. i need one for my rotax countershaft nut too.
    you can get small quantities from mcmaster.com, just search for "nord lock".
    Mike
    #20