Output shaft problem

[pov]

Hi all

My '99 sv was going well racing at Anglesey on Saturday until I came in after race 2 and noticed the front sprocket nut had worked loose. No problem I thought, I'll tighten it. Wouldn't tighten flush. OK, I'll take it off an have a look. Wouldn't come off. It moved to the end of the thread, but even with heat, an 8 foot bar, with 4 lads holding the bike down, it stopped and wouldn't budge either way. The race weekend was over, and now, having come home and got the nut away via grinder and chisel, I'm left with this.



Any ideas how I go about sorting this? Apart from getting a new front sprocket which I suspect might be the least of my worries...

Thanks in anticipation.

[Sid Squid]

I suppose it's just possible that if you tidy up the thread then the nut might just hang on to the remaining metal, but I don't hold much hope of that. Sadly the only way to fix it properly is to replace the output shaft.

Some shifty sods have been known to weld the sprocket on, this will work in the short term obviously - but it's only putting off the inevitable shaft replacement.

It may be possible to repair the shaft, perhaps metal spraying and re-machining, but I rather doubt this will be cheaper than replacement, and I can't see anyone doing it in place anyway, so the motor will need to come out and apart, in which case a replacement shaft, or even complete S/H gearbox will be simpler.

[yorkie_chris]

Gearboxes break very rarely so a replacement shaft shouldn't be too hard to find.
To the point where I wouldn't see welding it, machining and re-heat treating it as sensible.

More practice required with the grinder!

[punyXpress]

Cut a new - obviously smaller - thread on the shaft & use a spacer against sprocket?
OBVIOUSLY NOT an engineeer's solution!
Is the amount of spline sticking out normal?

[pov]

Thanks chaps, as I suspected and your advice confirms I'll be having the motor out this winter, want to do a proper job rather than a weld/re-thread...

Chris, you're right about the grinder though the damage to the sprocket was intentional - we knew that had to be sacrificed in order to try and get the nut off - there just wasn't the room to get the arc of the grinder in there without cutting the sprocket. I got the chain well out of the way and was gambling that someone in the paddock would have a spare sprocket for me, and they did, but we just couldn't shift the nut while we were there and as it turns out wouldn't have been able to re-attach a new sprocket anyway.

The nick on the end of the shaft - well yes that was my shaky-handed work! I hope I'm not so wayward when I'm on track!

[embee]

Obviosly the others are right, the fix is a replacement shaft.

However, I'd suggest that if push came to shove and you wanted to use it short term, there might be a fix.



The nut has a deep counterbore to ensure the thread doesn't bottom out. You could turn a sprocket nut down (assuming you can't find another suitable plain nut with the right thread) so that it uses the region of undamaged thread, obviously clean up the end of the shaft so the new nut clears it.

You would need to be very careful about getting as much engagement as possible on the remaining thread without bottoming it out. It might be easier to turn the nut to be effectively a plain nut and then measure the distance between it and the sprocket when it's bottomed on the thread run-out, then produce a spacer say 1-2mm thicker to ensure clamping.

It looks like there's enough good thread to make it work (maybe).