The longer I own this bike, the more I'm convinced that Yamaha made its sprocket retaining nuts out of cheese.
Twice, I've had these come loose now. Luckily, in both cases the locking nut did its job. When looking at the self-loosened nuts, which had worked themselves loose despite being set at the right torque tightness, the threads were mashed (on the nuts). But the drive shaft was fine.
I've had enough of this fault, and will be using a beefed-up Fazer sprocket nut instead in the future. Fazer owners had the same problem, and a company responded by manufacturing a surrogate which was 3mm thicker (and probably tempered to be harder). One nice thing is, the Fazer replacement is a quarter the price of a replacement XJ600 sprocket nut. I've therefore ordered one, will try it and report back.
