Hi fazersharp, here's my experience after your request for more info from 98 owners.
I've not long had my 98 600 and as yet haven't ridden it as I'm in the process of giving her a tidy up.
After reading about the possible issue relating to the original 9mm nut, and finding that the chain on sprockets would benefit from renewing, I opened up the sprocket cover and found the mess shown in the pics below.
It looks like the front sprocket has never been touched (bike has done about 42k
) and still has the original 9mm nut. Thankfully, it was still tight, the retaining washer folded over on 2 sides.
Flattening off the washer I undid the nut which was a little stiff but not too bad, and the threads were all intact. I'd say that it would never have come loose on its own. I will, however replace it with a new 12mm nut and retaining washer and a little loctite to be on the safe side.
Obviously, I'll be cleaning up the gunk before fitting the new c&s kit
Steve