front sprocket nut

[Fazerider]

Here's some competition for the later-bikes-have-an-undersized-shaft theory:
a) the nut was underspecified
b) early Fazers had it glued on with a ridiculously strong threadlock (hence the need for a scaffolding pole extension on the socket to shift it )
c) Yamaha realised they'd made the bond too strong and switched to a different compound on later Fazers which proved to be inadequate
d) when owners/mechanics replaced the sprocket, the thread was contaminated with oil and they didn't use high-strength superglue hence subsequent failures on early bikes
e) Yamaha's instruction to dealers regarding shaft replacement relied on a measurement of the diameter of the threaded portion not because they'd made some too small, but as a means of determining whether the thread had been too badly damaged by the loose nut and/or sprocket.