Advice needed

I've just finished fixing my mate's xs11. He had been going down the motorway when the drive train started making an awful noise - can't reproduce what he actually said, my wife's in the same room!

On rotating the back wheel, I found that there was a tight spot so I undid the 4 bolts fastening the driveshaft to the middle gear. The tight spot had gone so I took the middle gear out and here's what I found.

The nut holding the coupling to the middle gear had sheared.

The bike has only done around 15,000 miles and the middle gear has never been touched apart from oil changes.

All I can think of is that it was overtightened during manufacture.

Has anybody out there any other theories?

Mike Farnworth
 

I've had a couple of replies and those who've had this problem, have also had wear in the splines in the rear drive. Watch this space for further developments!