Dreaded front sprocket nut issue

[markus2066]

Hi All,


Its not been a great weekend for my bike. Whilst replacing the chain and sprockets I noticed that all the thread on the nut and shaft holding the front sprocket has been polished clean off whilst its been rattling/spinning on the shaft for an age. Seems that all that was stopping it coming off was the casing.


I need the bike to get to work so a short term solution has been having the shaft spot welded on the top of two opposite splines to stop it coming off, not the proudest moment for mechanics.


Does anyone know of any better solution to this issue which isn't rebuilding the engine or getting a used engine?


Also what needs to come apart if I were to go down the rebuild route, I'm guessing.......engine out, sump off, casings off, clutch out, alternator magnet off? then I can pull out half the gear box and replace one side? Any ideas roughly which (if any) new seals and gaskets would be required?


Trying to work out the best course of action, without breaking the bank......only paid £1100 for the bike.

[fazersharp]

I think someone on here has welded and also i someone tapped drilled and tapped a thread in the end of the shaft and did somthing.
Sorry not much help but perhaps you could help me as over the years on this forum I have not come across any 98 reg with the issue unless the sprocket has already been changed before. Do you know if the sprocket has been replaced before - how many miles has it done.   

[Fazerider]

It's a major job to replace the shaft: the engine has to come completely apart to get the casings split, you can't just post a new shaft in from the side.
When mine failed I resorted to welding the nut on and ran the bike for another 50,000 miles like that. The Scottoiler helped, I only needed to change the C&S once in that period… it feels a bit barbaric to use an angle grinder to remove the weld, but needs must.

[darrsi]

I think someone on here has welded and also i someone tapped drilled and tapped a thread in the end of the shaft and did somthing.
Sorry not much help but perhaps you could help me as over the years on this forum I have not come across any 98 reg with the issue unless the sprocket has already been changed before. Do you know if the sprocket has been replaced before - how many miles has it done.

The sprocket has nothing to do with it, it's presumably that poxy ill fitting 9mm nut.

[fazersharp]

I think someone on here has welded and also i someone tapped drilled and tapped a thread in the end of the shaft and did somthing.
Sorry not much help but perhaps you could help me as over the years on this forum I have not come across any 98 reg with the issue unless the sprocket has already been changed before. Do you know if the sprocket has been replaced before - how many miles has it done.

The sprocket has nothing to do with it, it's presumably that poxy ill fitting 9mm nut.

The issue when I am talking about "has the sprocket been replaced" is that so far no 98 with the original factory fitted sprocket has had the issue that has been reported on this forum.The problem with a 98 that has previously had the sprocket replaced which the nut comes off is that we do not know if it was properly torqued up by the person replacing it.
I asked about the mileage because that may give a clue as to if the sprocket has been replaced before markus2066 got the bike.

I was told by yamaha that 98s were not effected, other contributors on here have pointed out that on the effected bikes its the output shaft that is not to spec and is too small.
My pet theory is that the original 98s shaft was only produced in 1 factory and it was after 98 when production stepped up that other factory's were brought online to produce the shafts which were undersize. So all 98s are ok and thereafter its a game of chance where your shaft was made as to whether or not you have the issue.
This bares out as not all bikes after the original 98 have the issue and after a certain year they don't either, how were the later bikes in factory sorted, did they just put on a bigger nut or was it the same nut on a properly spec size shaft ?.
Also some people have reported needing a hefty extension bar to get the old thin one off which clearly wasn't going to fall off anytime soon.

This being said - anyone who is fitting a new sprocket should use the upgraded nut.
Here is my 98 thin nut, no I am not complacent about it and I am overdue to check it again, and I do have an upgraded nut under my seat ready. 

[Punkstig]

I think someone on here has welded and also i someone tapped drilled and tapped a thread in the end of the shaft and did somthing.
Sorry not much help but perhaps you could help me as over the years on this forum I have not come across any 98 reg with the issue unless the sprocket has already been changed before. Do you know if the sprocket has been replaced before - how many miles has it done.

The sprocket has nothing to do with it, it's presumably that poxy ill fitting 9mm nut.

This bares out as not all bikes after the original 98 have the issue and after a certain year they don't either,

Wrong

[darrsi]

I think someone on here has welded and also i someone tapped drilled and tapped a thread in the end of the shaft and did somthing.
Sorry not much help but perhaps you could help me as over the years on this forum I have not come across any 98 reg with the issue unless the sprocket has already been changed before. Do you know if the sprocket has been replaced before - how many miles has it done.

The sprocket has nothing to do with it, it's presumably that poxy ill fitting 9mm nut.

The issue when I am talking about "has the sprocket been replaced" is that so far no 98 with the original factory fitted sprocket has had the issue that has been reported on this forum.The problem with a 98 that has previously had the sprocket replaced which the nut comes off is that we do not know if it was properly torqued up by the person replacing it.
I asked about the mileage because that may give a clue as to if the sprocket has been replaced before markus2066 got the bike.

I was told by yamaha that 98s were not effected, other contributors on here have pointed out that on the effected bikes its the output shaft that is not to spec and is too small.
My pet theory is that the original 98s shaft was only produced in 1 factory and it was after 98 when production stepped up that other factory's were brought online to produce the shafts which were undersize. So all 98s are ok and thereafter its a game of chance where your shaft was made as to whether or not you have the issue.
This bares out as not all bikes after the original 98 have the issue and after a certain year they don't either, how were the later bikes in factory sorted, did they just put on a bigger nut or was it the same nut on a properly spec size shaft ?.
Also some people have reported needing a hefty extension bar to get the old thin one off which clearly wasn't going to fall off anytime soon.

This being said - anyone who is fitting a new sprocket should use the upgraded nut.
Here is my 98 thin nut, no I am not complacent about it and I am overdue to check it again, and I do have an upgraded nut under my seat ready. 

Whether the sprocket has been replaced or not, or whether the nut has been under or over torqued, the fact remains it'll still have the same shaft and the same 9mm nut (presumably, he doesn't actually say)
And it's a 98/99 bike, so if you want to believe Yamaha then go ahead, but i would imagine they'll say anything to fob you off, especially on bikes that are this old.
And don't be fooled by your nut being still in place, mine did the opposite to coming loose and fused itself on instead, so it was never going anywhere and took my mechanic ages to get the bugger off.

[fazersharp]

I think someone on here has welded and also i someone tapped drilled and tapped a thread in the end of the shaft and did somthing.
Sorry not much help but perhaps you could help me as over the years on this forum I have not come across any 98 reg with the issue unless the sprocket has already been changed before. Do you know if the sprocket has been replaced before - how many miles has it done.

The sprocket has nothing to do with it, it's presumably that poxy ill fitting 9mm nut.

This bares out as not all bikes after the original 98 have the issue and after a certain year they don't either,

Wrong

Which bit.
I have to say its all my own assumptions and theory based on what I have seen and read over the years on this forum and based on my own bike, some people have it some don't and as of yet no 98 has it - remember as above no 98 factory original untouched.   
This is why I am interested in marcus2066 as his avatar say 98-99 bike and he has the issue 

EDIT
Juts read the posters other posts and he has a 99 with 39k miles and reported the sprockets looking new.
So its not a 98 and the sprocket looks like has already been replaced.
So yet again I cannot find any evidence to disprove my theory.

[fazersharp]

Whether the sprocket has been replaced or not, or whether the nut has been under or over torqued, the fact remains it'll still have the same shaft and the same 9mm nut

Its not the torque or the nut - its the shaft that is wrong. The talk about torque is that if someone has reported a 98 with the issue but then it transpired that the sprocket has been replaced before and so it canot be rulled out that the last owner did not re torque it properly.

No I don't trust yamaha but no one has reported a 98 with the issue - on an original untouched factory fitted nut.

[Paul]

I
I reckon the reason why the problem with the front sprocket nut exists is

 
Minor play between the sprocket and shaft, due to shaft or sprocket being machined out of tolerance.


Nut tightened up on the sprocket.
Sprocket moves fractionally back and forward, and because the nut is tight to it, so does the nut.
Nut threads wear out as do the shaft threads.
   
Possible solution:  Grind down the over-lap of the nut so it tightens up on the shaft and not on the sprocket.
In essence it should be flush with the sprocket but not tightening up against it.
Ideally make sure there's a layer of grease between nut / security washer / sprocket.

[Punkstig]

Last of the carb bikes still have the issue- not all were given the bigger nut which is the cure!

[markus2066]

Thanks for the input guys, good to know i'm not alone with the welding solution. The guy who did the weld initially suggested welding the nut on but the way I see it is if the weld on the nut cracks it will fly off, if you weld the top the splines and it cracks its still not going to get over the spot weld and there is one on each side so.....not ideal but hopefully safe enough.


I had considered getting a scotoiler as mentioned as I've heard this can double the life of your C&S and would be alot longer before I would need to worry about it. Certainly glad to hear your 50k miles on, makes me abit more confident about it.


I'm usually a big fan of doing things right but when you need the bike daily and this happens there is not a lot else you can do i guess. I was worried people were going to say the weld job was dangerous, also abit of a confidence boost.


My bikes a 99 and it didn't have the wide nut on, I had heard something about this issue and was planning on getting one if I found it didn't when I did the job.......bit late now i guess.

[Deefer666]

I have welded a sprocket on as a stop gap to keep a customer on the road, but he came back at a later date for me to swap out the output shaft. Its not a difficult weld, the difficult bit is getting everything clean enough and de-greased for a decent weld to take. If you are planning this, remember to disconnect the battery and remove the main earth lead from the rear RHS of the engine. 

[markus2066]

Sounds like an excellent piece of advice Deeher, the welds already been done and everything was still connected.......sees ok. Will be useful point for someone searching posts on this issue.


Regarding cleaning up for welding, I took the front sprocket off and cleaned up two splines with a file so had some good clean metal and a new sprocket so hopefully strong enough. I'm going to take the cover off again in a week or two and see how its getting on, definitely something i'll be keeping an eye on.


I am a big fan of my Fazer so likely i'll do something more permanent about this at some point.