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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 28 Aug 2009
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Front Sprocket Explosion

I just have to post this. My 86 XT 600 is an awesome bike - hands down the best Enduro I've owned. I've figured out the carb trouble, suspension woes and minor squabbles. But there was this racket coming from the rear tire area that was making me nuts. I re-aligned the tire, checked bearings, chain, rear sprocket, spokes but still had a 'chain-slap' noise that wasnt going away until.... I geared down to third to turn right, made the turn, got on the throttle when BANG and parts went flying. My rear locked up until the chain snapped, I went into a speed slide, bike died but I held onto it, coasting to the curb. Behind me lay strewn about was pieces of plastic and metal. The front sprocket snapped in half at about 60k. Along with it the chain is KIA, chain guard, chain guide and sprocket cover. $250.00 Can.

The noise was from the sprocket bending because of the cracks and making the chain whip.
The reason it snapped is evident in the pictures. Yes that is failed weld. Who welds a sprocket onto a tranny shaft??
This is a classic case of 'buyer beware' and things you do when drinking too much . I ve saved the sprocket, will put it in a sock and nail the dumb SOB in the head with it. I could've been killed had this happened on the freeway.
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  #2  
Old 28 Aug 2009
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wow that was a close one. glad your ok. ive heard of this welding the spocket on before but not seen it, the sob should have told you (if he knew) as its a pretty naughty thing to do
ive just brought an 85 model tenere and did check the front sprocket as id recently read about this somewhere else. guess youve got a rebuild on your hands now, is the shaft totally knackard? good luck
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  #3  
Old 28 Aug 2009
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PO 's do strange things.
Like installing dry charged battery dry ( no acid).
Fitting split link the wrong way on the wrong side.
can sometimes take years to get a bike back to specification with 'everything' fully functional.
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  #4  
Old 29 Aug 2009
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Wow. I have not seen anything like this before. I’m glad you are OK.
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  #5  
Old 29 Aug 2009
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Ya thanks. I got lucky. As for the tranny shaft - its pretty messed up but i have a close machinist friend. He's massaged the splines, cleaned it up with small die grinders and we're waiting for parts to arrive. He will be able to fit the new sprocket on with the appropriate retainer, he's quite good.
NO WELDING.
I'll post pics this week of the before and after of the shaft.
Also my friend says that the sprocket needs a small amount of free play on the shaft in order to find a center with the rear sprocket and float with the chain. Hence the retainer that clips into a groove on the shaft that bolts to the sprocket. It just makes sense. Welding it did a few BAD things: heat warped the sprocket, no free play, destroyed the integrity of the output shaft and most important put lives at risk.
I don't know if the guy new about it and held it back when I bought the bike. I'd like to think not. Even I missed that one, so I can shoulder some blame as well.
just please all you new and experienced riders preventive maintenance and regular inspections could save your life. Just Do It.
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Old 29 Aug 2009
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Quote:
Originally Posted by pulpbleacher View Post
Also my friend says that the sprocket needs a small amount of free play on the shaft in order to find a center with the rear sprocket and float with the chain. Hence the retainer that clips into a groove on the shaft that bolts to the sprocket.
Not sure I understand this. The manual on my bike says the sprocket nut should be tightened to 110NM, and there's a lock washer on there too. How can the sprocket 'float' or have any free play? Mine is 100% solid on the shaft and, given the work it does, I'm glad it is
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Old 29 Aug 2009
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Whilst HONDA seem to have a FLOATING front sprox arrangement on many bikes, In my experience, most bikes have a fixed front sprox - as BlackDogZulu says, torqued up real tight.

I did have a front sprox come loose on me old (1978) Z1000 once....... I had noticed the chain snagging a bit when I pushed it out (backwards) from me garage, but thought nothing of it.... Then a few miles down the road, after a blast down the dual carriageway.... I stopped at a red light... Pulled away & then the bike just revved - no drive. The front sprox had loosened & just fell off.... Luckily no damage to me or the bike & the nut & bits were still under the casing cover

Good thread this by the way
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Old 29 Aug 2009
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That explains the friend's remark then - he was thinking of Hondas. When you think of what the front sprocket has to do, it's one thing that I really check twice and then twice again before I put the covers back on. That nut is on bloody tight, and a bit more
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Old 29 Aug 2009
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Blackdogzulu - here's the deal. Yamaha made many different parts for their bikes over the years. My 86 xt 600 was manufactured for North America. Some parts on the bike are different than those in Europe and elsewhere. For instance: Yamaha no longer offers a conversion from off-road to S/L for North America, also Yamaha is no-longer offering dual-sport bikes for North America. Our Gov't has changed the rules and ****ed us over.
Now about the shaft and sprocket - the shaft on my bike is splined half way down the shaft leaving a small space with no splines. the splines continue on at the base of the shaft before it disappears into the case. Therefor the sprocket resides in the middle of the shaft with only the two outside edge making contact with the splines. The retaining clip attaches to the spline gets turned to line up with the holes in the sprocket and bolted down. There is no center bolt and the shaft is not machined for one. The sprocket is in contact with both splined areas. the 'free-play' is so minuscule it cannot be detected by using your hands. Only under torque loads will the 'float' occur. Just as the rear hub/sprocket has rubber torque wedges to absorb some twisting force, the front sprocket does as well. hope this helps you understand the concept. As I stated earlier as soon as the parts arrive I will post pics of the shaft for you to see - before assembly and after.
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Old 29 Aug 2009
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I'm not sure where the change over comes, but i think early XT / Tenere's use the offset retainer method for the sprocket and later ones use the bloody big nut approach.

Both my 1VJ tenere and my szr 660 (which has an xtz660 engine) use the bloody big nut plus a tab washer; my clymer manual shows the xt and tt as having the offset retainer, so i suspect that applies to the earlier 43L and 55W models - no doubt people with them will confrim that.

Hondas seem very fond of the offset retainer which has the advantage of not requiring the whacking torque of the bloody big nut. At the end of the day, there's almost no lateral force on a well aligned chain, so the offset retainer is fine.

Personaly i prefer it as many moons ago, the bloody big nut on my gsx250 came undone after a zoom up to birmingham (I was young and forgot the tab washer...). I ended up getting the nut welded to the shaft end to get me home and having to dress the whole thing back up when i rebuilt it....

We live and learn - it's good to see that pulpbleacher is still living tho'... nasty one!!
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  #11  
Old 3 Sep 2009
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Finally received the parts and a couple hours later; I'm on the road. About an hour of using files and small grinders we trimmed the shaft so the sprocket would fit properly and safely. The retainer went on great and locked it in place. Installed the new chain and the rest of the parts.
What a relief we were able to salvage the shaft. Pictures to follow.
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