Picked up my new nut and gasket yesterday from Poole Yamaha and was asking about why they come lose and he said it was due to after market sprockets being thinker than the originals so it kind of makes sense now. And it only meant to affect one in every 1000 fazers. So doing mine just in chase 
Steady on guys, this all depends on how you read the word "thinker" in the above post. Is it meant to be "thicker" or "thinner"?
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I very much dought it is thicker as all the original Yamaha sprockets I have measured have been between 9.58 to 9.64mm, which is right on the top limit of becoming a tight fit in a 530 size chain. Theoretically a thicker sprocket (or lock washer) would give slightly more thread engagement with the old nut and very slightly less with the new nut. I have never measured aftermarket sprockets but I suspect they may be a bit thinner.
If a replacement sprocket is thinner then the guys comment does make a little bit of sence with the old nut, although he is still barking up the wrong tree. The old thin nut overruns the shaft thread by about 1mm and a thinner sprocket would make it worse. Within reason it would not have much effect on the new 12mm thick nut.
The real issue, which Yamaha acknowledged, was that some bikes had gearbox shafts fitted that were below tolerance on the diameter of the thread. A minimum O.D. of 17.5mm was the figure given. (I have only measured three and the were all around 17.85mm O.D.).
In the distant past when this topic was lively (on the old forum I think) I did a drawing which would illustrate the above ramblings on thickness. I will now attemp to post the drawing on here.
