...The reason for it is that some bikes were produced with very slightly undersized (diameter) output shafts...
I've heard this said before. If this is true, does anyone know what diameter it should measure so we could check?
Yes, I've seen the same claim many times, though all from the same source and it is only a theory.
17.8 mm is the typical diameter measured across the threads, Yamaha say to change the shaft if it's less than 17.5 mm. I've not heard of anyone measuring theirs and finding it undersize unless it's previously been battered to death by a loose nut and/or sprocket. Shafts being
manufactured undersized will remain a myth until that happens.
The nut is of such soft steel that the threads on the shaft often survive it coming loose. The real damage is done if the nut falls off the end of the shaft, then the sprocket can move onto the threaded portion and chew it up. Later Fazers are at much greater risk of this because they lack the damper plate on the inside of the sprocket cover which tends to prevent a loose nut, particularly the 12mm one, from falling off the end of the shaft.
Earlier Fazers also had the factory nut put on with a ridiculously strong threadlock, anyone who's undone one can tell you that it requires a very long lever, a lot of force and it sounds like a gunshot when it finally lets go. I was convinced I'd broken something!
The nut doesn't necessarily rotate to become loose. Several times when it's happened to me the washer was still engaged with both the nut and the splines meaning the nut has managed to "hop" a thread. There has also been excessive wear on the inside edge of the sprocket and I suspect the sideways hammering from the chain might be the cause of the nut becoming loose in those cases.