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Thanks, Greenbat, taken that onboard.Bike is stripped of everything apart from handlebars and wheels.With oil drained I might try turning the sucker upside down, like you do with a pushbike and have a close look at what is going on, give it a right good soaking with penetrant over a week then hit it with some heat.
Can't see the problem myself. Ok the bolt may have a odd thread, very fine IIRC but so long as you change the nut as well as the bolt and the length is right you can use any 10mm bolt. I replaced all my engine bolts with stainless 30 years ago, best bet is to make sure you grease them BEFORE you fit them as it stops the corrosion starting and ending up like this.