So it's Chain and Sprocket Time

[Dead Eye]

D.I.D is a good name though most on here recommend the Heavy Duty version instead (circa £90-100~ last I had seen)

The uprated OEM nut can be bought from most Yamaha dealers for under £5 or you can buy them online but costs do vary. I got mine from Wemoto as I couldn't be bothered to try and find my dealer plus it looks like the price has dropped on their site as well - it cost me around £15 last time (incl. postage) but has dropped to £6~ (excl. postage) now

http://www.wemoto.com/parts/picture/yh-90891-10124/