I don't know.. the nuts look good old stock to me, it seems they were never removed. I bough the bike used but have a good history of maintenance and parts bought by the previous owners. The nuts were pretty hard to remove and some got easely rounded in the process, so it may be aluminium?

Well anyway i cut the nut and could remove the sprocket. The stud took quite a beating, but after removing the broken threads of the nut, i can tighten and remove another nut on the stud (there is a hard point at the beginning but then it goes on ok).

Ok so ready to order the swing arm guard and nuts. Should i order yamaha nuts at 3€/nut, or is it some standard stuff i can find elsewhere , like any self tightening steel M12 bolts?

thanks again