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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 12 Jun 2010
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XT600 Front Sprocket Disaster.

I've got a 1985 XT600. Up until a couple of days ago it's been a super bike and competely reliable. Its done about 35,000 KM (spent the first part of its life in Italy so has a KM speedo). Then when I was pulling away from the last roundabout on the way to work I suddenly lost all drive. Engine running, gear box still OK but open the clutch and nothing happened. I thought at first that something in the clutch had gone but was horrified to find that the front sprocket has completely worn away the splines on the gearbox output shaft. Before I took the sprocket off I had thought it was the other way round so felt sick to see the shaft worn round.

So I'm going to have to drop the engine out and split the cases but would like to establish what has caused this to happen. Wonder if anyone else has had this happen? The chain was a new heavy duty one and kept well oiled and I fitted a new retainer plate with the sprocket, which was a pattern part rather than OE. The chain was also kept tensioned but was by no means tight.

Any thoughts and suggestion gratefully recieved.
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Old 12 Jun 2010
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I imagine, like an XT350 i once owned, it had a bit of abuse from previous owners with wheelies and the like. same thing happened to me
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Old 12 Jun 2010
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The Rounding off of the splined main shaft did not happen over night, in fact I'm quite sure it would have taken years for that to happen, probably could have been spotted two years ago or longer.

Even a casual glance during routine maintenance would have turned this up early. Did you Ever change countershaft sprockets? Doing that simple job would allow you to see the wear up close ... not that there is really anything to be done ... except WELD the sprocket on the shaft .... which I have seen done!

Some new aftermarket sprockets now are using some magic super hardening process to allow long life. The problem is that the surfaces are SO HARD they destroy chains early and maybe even wear out your splines on the shaft. That's another reason I prefer OEM sprockets.

But your bike is a 1985. That's 25 years of use/abuse I'd say you've done quite well. Sell that Old Boiler for parts (or set it ablaze) and buy a newer bike. Move on. Not worth splitting the cases (IMHO) unless you can perform all the work yourself.

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Old 13 Jun 2010
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Thanks for the replies.

To clarify, the sprocket was put on new, with a new retainer plate within the last year or so. Mileage since then has not been great - I have probably adjusted the new chain fitted at he same time twice. At the time the sprocket was fitted the shaft was fine. There was backlash between the sprocket and the shaft but that seems to be common - i've certainly seen the same with other bikes, including another XT of the same model and age.

I know that '1985' suggests the bike is an absolute dog, but actually it is very original, unabused and in excellent mechanical condition. When it came into my hands about five years ago even the cadmium plating on the fasteners was still bright. I certainly haven't abused it. I'll fix it myself.

I'll opt for a super hardened sprocket as the most likely culprit. That is still in fine condition... I think that using OE sprockets is good advice.
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Old 13 Jun 2010
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I always lube up the shaft where the CS sprocket sits, dont know it that could have changed anything, its just something I do.
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Old 13 Jun 2010
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Causes:

Running the chain WAY too tight sometime in it's life.

Using cheap, crappy sprockets that don't fit snuggly is the main culpret in my experience.

Also people not locking the sprocket properly with the nut/tab etc and letting it drift on the spline.


The ONLY thing you can do is replace the shaft or weld a new sprocket on until you have the parts/cash/time to do the replacement.
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Old 13 Jun 2010
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I've had this happen twice on a DR600, never used cheap sprockets or ran the chain too tight. If the chain was too tight, you'd experience issues with suspension and output shaft bearings wearing before splines wearing away.

I've also seen it happen on a KLR 600 Tengai, I think it's down to mileage & the power pulses that come from running a big single.

A full strip down & replacement of the output shaft, sprocket & fasteners is teh only only option. Welding the sprocket to the shaft is a temporary measure that will work until the sprocket itself needs replacing.
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Old 14 Jun 2010
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Quote:
Originally Posted by DRPMC View Post
Thanks for the replies.

To clarify, the sprocket was put on new, with a new retainer plate within the last year or so. Mileage since then has not been great - I have probably adjusted the new chain fitted at he same time twice. At the time the sprocket was fitted the shaft was fine. There was backlash between the sprocket and the shaft but that seems to be common - i've certainly seen the same with other bikes, including another XT of the same model and age.

I know that '1985' suggests the bike is an absolute dog, but actually it is very original, unabused and in excellent mechanical condition. When it came into my hands about five years ago even the cadmium plating on the fasteners was still bright. I certainly haven't abused it. I'll fix it myself.

I'll opt for a super hardened sprocket as the most likely culprit. That is still in fine condition... I think that using OE sprockets is good advice.
Good news that you can do the work yourself!
Not to doubt you too much but I do find it very hard to believe that your splines did not show wear only a year ago ... unless you've done 50,000 miles in that year. A super hardened sprocket could wear it down but NOT in a year. I was born at night .... but not last night. Spline wear takes lots of time. If, in fact, you inspected it when you replaced the countershaft sprocket then all I can think is that you missed the wear that was already there. Maybe you had splines but how deep were they? And how were the shoulders of the splines? Also, that back lash you felt is a sure tell tale of this sort of wear. Also check your Cush drive rubber inserts. Worn ones really let the drive line get sloppy.

Anyway, best of luck with the tear down. Not an easy job by all accounts.
Hope all goes smoothly!
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Old 14 Jun 2010
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I,ve just replaced my 5th gears which involved a complete strip down,never have done this before and found it an interesting job......take the bull by the horns and just do it! take lots of pictures and look on you tube at this YouTube - Yamaha SRX 600 Engine Disassembly I its a bloke stripping down a yam srx 600 which is almost the same as the xt600 and gave me loads of confidence to try it.

If you have any problems just write in to this site and people will help.....
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Old 24 Jun 2010
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You've been countershafted!

I had exactly the same thing happen to me on an old 88 1VJ. At the time, I was exitting a long tunnel in Italy, with three lanes of mad Italian drivers on all sides - not cool! Luckily my chain did not lock up and I free wheeled to a halt.

This is not caused suddenly, it happens over a long period of time. I think in the pre 3AJ teneres the front sprocket attachment with the two retaining bolts is a weakness, in the 3AJ and later models they replace this with a single large bolt to keep the sprocket on.

To fix this properly, you will need to replace the countershaft (I think it is called) - a fairly big job, as it means taking the bottom end apart.

Good luck!
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