Loose Sprocket Nut

if I were you I would I would just tighten it as best you can and maybe use a spot of Loctite then Re-tab. as long as it is tight it will be fine. I do not think the torque is that critical in this situation.

 

Make sure the nut is VERY tight!

I have had this happen on a Yamaha 600 thundercat and put it down to high revving down changes (think reverse torque effect). I'd changed the sprocket kit 6 months before and the nut was definitely tight however.......
Returning from an MOT and approaching a roundabout the front sprocket fell off. That wasnt immediately apparent but the slack in the chain was.
Upon inspection the tab washer was present along with the sprocket inside the casing. The tab washer was still tabbed over but the nut was totally absent.

 

What @Eldon said ! Tight Tight Tight as , you are putting *#+ horse power through the sprocket in various gears ! 1st gear multiplies the torque and horse power exponentially in comparison to top gear !!!

 

The nut is to keep the sprocket in place . the torque and horsepower should surely be going through the splines on the shaft and sprocket??

 

If it's not tight enough it will rock and roll (wobble) on the shaft against the nut and washer , this is how wheel nuts come loose and work their way off. Wheel nuts have a required tension and must be done up in a specific pattern.

 

It’s not just Triumphs, this is very common on Yamaha Fazers, the same engine as the Thundercat that Eldon mentioned.
Yamaha did a recall on them to fit a thicker nut which seemed to fix the problem

 

Freck my Drz400 suffered the same, hence why used a bit of Loctite on the nut and the tab.
I still don't think that 132nm of torque is needed on the nut though.

 

Looking around the Yamaha Thundercat forums after my sprocket fell off it turned out to be a common issue there as well. In fact Yamaha came to realise it proven by the fact that it released an amended nut with two more threads or so. I believe the real issue was the layshaft was too short on threads but they weren't going to admit to that were they?
The new nut came right flush to the shaft end.

There were a number of "owner fixes" mentioned including welding the sprocket directly on to the shaft. Imagine buying that as a secondhand bargain?