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Low Flyer.
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Discussion Starter · #1 ·
I have endless issues with the bolts on my back sprocket coming loose! Twice now, I have managed rip the sprocket off the hub and had to replace almost everything on the rear. No one I've spoken to seems to know what the issue could be or even where to start looking.

I tighten the bolts after every few laps on the track. after the first hub replacement, i even discarded the nuts and fitted nylon threaded nuts with lock tight, this seemed to hold for a few months, with continuous checking. But eventually they too came loose. After continuous replacing of the bolts and one too many laps between checking them, I managed to destroy the hub again. This is the second time in a year.

I bought the bike brand new from a dealer and for the first few months never had a worry. And the first time it happened, no one had ever removed the original Yamaha bolts.

After all my looking into the matter, the only thing left to check is if the swing arm is not the cause. Maybe it has been bent and is now transferring pressure down the chain to the rear sprocket.

Does anyone have the same issue? Or even some advise on what i could check?
 

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Life: Tragedy or Comedy
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Are you using a torque wrench to tighten the nuts and bolts? I've replaced a couple of different rear sprockets now and not had any issues.
 

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Low Flyer.
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Discussion Starter · #3 ·
Yes i have and once the bolts have come loose, the torque is actually too high and snaps the bolts. Obviously once they have come loose, the sprocket starts moving around and weakens them. I have had many bikes over the years and changed many sprockets, and also have never had an issues whatsoever.
 

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once in a while we have someone come in the shop like yourself, and on a good hub new bolts and nuts and always use the flat washers they do help spread the stress and use locktite red and that most of the time cures the problem , but sometimes a talon hub is required to cure this , it depends on the way you ride , hope that helps.
 

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Sand Expert
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I like that idea. If these bolts are not flange bolts they using some. They have a higher torque and strength value. The underside has a line that is angled to give it a bit more bite. make sure the bolts use all the threads too. Use the red, if there is room use the stover lock nut on them. Use a straight edge to check the mating surfaces too. This is an unusual occurance. Good luck
 

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time in a bottle
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Its a bit of a stretch , but check for worn swingarm bearings .
check for movement of the rear wheel and swingarm .

A misalignment could cause a problem I guess . so it is possible that a side to side movement of the swingarm could break a sprocket loose .

again it is just something to check not necessarily the answer.

hope it helps.
 

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Low Flyer.
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Discussion Starter · #7 ·
Thanks guys, i appreciate the ideas.
I will try them all out, 'coz i'm desperate to get this sorted out now.
I'm tired of hitting the track and not having the confidence.

When you say flat washers, your talking about on the nut side of the sprocket correct?

And flanged bolts? never heard of them, are they like tapered bolts?

The swingarm was my next item on the check list. will let you guys know though.

Thanks again.
 

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time in a bottle
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another thing to consider is the chain tension .

many people (including myself) have a tendency to over tighten the chain.

there should be between 40mm and 50 mm of slack in the chain.
this measurement is made with the bike on a centerstand and the rear wheel elevated.

which when you look at the bike sitting it seems like an extremely loose chain.

if you are jumping a dirtbike with a slightly overtight chain it could cause problems .
 

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Low Flyer.
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Discussion Starter · #9 ·
I did consider the chain tension.

I usually keep it to the required spec, but while riding i tend to get a lot of chain slap, as if it's too loose.

Does it sound like this could also be a cause?
 

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time in a bottle
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1,349 Posts
No I was thinking it may be too tight, but it sounds like you got the tension right.
I always want to tighten my chain too tight so I have to remind myself that the chain needs room for wheel travel .

I have never had the problem you have described. years ago Yamaha had a problem with the sprockets and hub breaking , but as far as I know that was ten years ago.

any chance there is an alignment problem? sprockets, guides , spacers

any chance any of these parts are misplaced or missing ?
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Low Flyer.
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Discussion Starter · #11 ·
I see what you're getting at David.
It's definitely not that anythings missing.

Alignment problem though, maybe. I've chatted to a few guys at the bike shops, everyone seems astounded by the issue. But one thing that seems to be the common comment, is that it might be the swing arm that's out.

The wheel itself may be aligned with the swing arm, but if the swing arm is bent, or something, from a previous crash, for example, then the chain might be causing some sort of sideways pull on the sprocket. And hence cause the bolts to come loose.

I think i'm just going to send the bike in to my regular guy and ask him to have a look. I'm just worried this happens again, not just the money issue, but it's gona play on my confidence on the track.
 

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time in a bottle
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yeah thats what I was getting at , if there is nothing missing or misplaced then I would check the swingarm because as I said it seems like an alignment problem . if the wheel is not tracking straight it would pull the sprocket out of it's roots so to speak .
 

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7c Forever
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Sounds like this is more or less sorted but I just wanna throw in there that I'm bad for overtightening the chain on dirtbikes too. I just can't bear to see 5cm of slack in it, I think forsure it's gonna fall off.

I pay the price in gearbox PTO bearings too.
 

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Low Flyer.
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Discussion Starter · #15 ·
Thanks for the advice, I'll keep you posted once I get it looked at.

I usually have the bike standing on the ground and use a four finger measurement between the swing arm and the chain, putting my pinkie at the back of the top chain guide rubber.
 

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Low Flyer.
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Discussion Starter · #17 ·
Finally, the mechanic figured out what the problem was on my rear wheel!
The previous workshop fitted a KX450 rear sprocket with slightly modified bolt holes, unfortunately, the id of the sprocket is 4mm bigger than the Yamaha one and with all the extra play, put all the pressure on the already ovaled bolt holes.
Can you believe it??

I can believe I didn't notice it.....Anyway, my bike is fixed and I'm going riding this weekend. And also started to fight the idiots that did this yesterday, I will make them pay!
 

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rear hub

I just had my rear hub on my 07 yz450 grenade on me yesterday at the track. the bike only has 12 hours on it. i bought it in feb 2010 brand new. I checked the bolts after my first couple rides and thought I was safe. as I was pushing my bike off the track a guy on an 06 yz450 pulled up and said the same thing had happened to him. Im curious about your final reply. you said an incorrect sprocket had been installed, but on your first post you said you bought the bike new. so, that would indicate an issue with the original yamaha parts, right? Anyway if your still having issues could you please post or pm me. I just ordered all new parts, but if this is going to be an ongoing problem I am going to bite the bullet and spend the money on a talon hub. because like you, I feel it really is going to kill my confidence at the track.
 

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Low Flyer.
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Discussion Starter · #19 ·
I did have an issue with the original sprocket bolts coming loose. The original sprocket had never been removed and the bolts only nipped here and there to ensure they were tight. But somewhere along the line they managed to loosen and once that happened they ovaled the holes in the hub and would never stay tight again.
Eventually, a few were lost and the rest of them took their vengence and ripped the hub to pieces.
The incorrect sprocket was fitted when i replaced everything. I later found out this bike shop had charged me for things that were never done, like oil filter, shock re-valves and head gaskets. Even using my new bike parts for other bikes.
It's no wonder the 'new' rear wheel didn't last.

Unless you are getting ripped off like i was, I don't feel it's an issue with the original Yamaha parts themselves. I have never heard of this type of problem before my bike and even people at the track that i spoke to, had never heard of it. It's more likely that it's something can will happen to 1 in a million and only regularly checking it and hopefully catching it in time will help.

Keep us posted VET and good luck.
 
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