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Discussion Starter · #1 · (Edited)
EDIT: Some more research (finding the right words to Google) shows that this is not uncommon wear due to the sprocket being harder than the shaft - I guess the lesson is, don't use steel sprockets.
The teeth that the sprocket fits over on my 2007 scrambler appear to be machined such that the teeth under the sprocket itself are much narrower than the other teeth. It's difficult to describe - the pictures show the shaft itself, the sprocket sticking out slightly so it fits properly over the teeth, and how far off the sprocket can sit when pushed fully on the shaft.

What's going on here? Is this how it's supposed to look? I've changed the front sprocket several times and never noticed it in the past, although that doesn't necessarily mean it didn't look like that, just that I never noticed. There's only been a couple hundred miles put on since the last sprocket change - could the sprocket somehow have worn down the shaft teeth due to improper installation? Am I missing a spacer between the shaft itself and the sprocket?


20210609_184516.jpg 20210609_184555.jpg 20210609_184605.jpg
 

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That's not good. The sprocket has been a poor fit and/or loose and has been chattering backwards and forwards under acceleration and overrun. I also suspect the hardening of the splines has failed because the external splines on the shaft should be harder than the internal splines of the sprocket, i.e. easier to replace the sprocket than the shaft which requires a full stripdown.
 

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Another thing to check is the rear hub cushion rubbers. My guess is that they've hardened over the years and are no longer protecting the drive train.
 

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the spline on your sprocket shaft is worn out and will need replacing. part #T1180901. if not, the remaining metal in the spline that engages the sprocket is going to shear and you'll lose power to the sprocket. there are no spacers missing

male and female splines have clearance of a few microns, (there's an engineering standard that gives a spec.) otherwise you would not be able to fit the sprocket, and if the large nut that clamps the sprocket is not tight enough chattering will occur between the two components during acceleration and deceleration. in other words, the sprocket is chattering back and forwards a few microns many times per second if the sprocket nut is not tight enough to clamp it up.
made even worse if the chain is too slack or the cush drive is as Dr. Shifty suggests. the constant chattering will wear the splines not matter how hard they are.

I'm guessing that the large nut has been loose and the only thing stopping it from unscrewing was the tab washer .
when I first replaced the sprocket on my bike the large nut was loose and some wear was on the countershaft spline, that's why I have subsequently made sure it is tight so no chattering wear can occur.

are you the original owner? maybe the original sprocket was loose like mine and most of the wear occurred then and it has been since held in place by a properly tightened nut that is clamping it. torque spec for the nut is 132Nm about 98 foot pound.
 

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Discussion Starter · #5 ·
Yes, I'm the original owner. This is maybe the 4th sprocket (the bike has nearly 40,000 miles on it, I've changed the chain and sprockets a few times), if anyone failed to tighten down the nut enough it was me.

Is it possible to slow any further wear just by making sure the nut is sufficiently tightened down? I've been contacting some garages, getting quotes for transmission shaft replacement and considering doing it myself - it would be the biggest repair I've done by far, but I've taken apart the engine to replace clutch springs, sensors, and valve shims, I've read through the manual and I'm confident I could do this if I took it slow and followed the manual - but any repair plans are complicated by the fact that I'm going to move to a different state at the end of July. Since this likely happened in the ~400 miles since I last changed the sprocket, I'm wary about riding it at all, but if I could be confident about getting a few dozen more miles out of it - to ride it to garages for more concrete quotes, or from wherever Haulbikes drops it off in my new city to my house, for example - that would make logistics a lot easier.

The cush drive is the original, and it's probably too old to do much good anymore. I changed out all the seals and bearings a couple years ago, but didn't even think about changing that out. It'll definitely get swapped for a new one before any real riding happens.
 

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I reckon if you tighten down the sprocket nut as per spec. ie, 132Nm you'll be ok as the drive then is being transmitted through the compression of the nut against the sprocket that is bearing on the collar directly behind it, rather than the spline itself.

when the nut is tightened to that torque it is almost impossible to loosen again without a rattle gun, so there is no way it is going to spin on the countershaft in the short term and shear whats left of the spline teeth.

my bike's the same age as yours, but with a lot more mileage and I haven't changed out the cush drive too. maybe I should as have no idea of what its like.

would be interested to here from other owners who have had to replace theirs
 

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This is a bit of a hack, but you might consider it as an emergency stop-gap.

Get another sprocket and have the outer half of it cut off so you are left with a smaller disc and the spline intact. Match the splines and weld the second one to the outer face of the original. This means the second sprocket is running on the outer edge of the shaft where the spines are in good condition. Tighten up the nut as usual.

Some sprockets come in this configuration to give more spine contact.

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Coincidentally I replaced the ( original ) gearbox sprocket ( with a 17t ) on my 2008 Scrambler yesterday and was surprised that the nut was barely tight . I was all prepared with a breaker bar and could have undone it with a 1/4 drive socket . Maybe a quality control issue with early machines ?

Phil
 

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I find it hard to believe that that is wear. Because the driving force is on the other edge of the shaft teeth. (*For left side chain.) Edit: Apparently it's a right side chain. Still looks like an unbelievably clean edge on the rear of those teeth. I'd wanna see the stock eom sprocket and another bike's shaft.
 
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