I bought a new nut from Draganfly. It fitted. After fitting I tried the SRM nut it was slightly better. I cleaned up the threads with a wire brush, then some gunk and moving the SRM nut on a few turns then back, never forcing it, I got it running fine and hand tight to the end of the threads. Must just have been dirty or slightly damaged threads. Strange the original nut fitted fine, the new original type nut fitted but the SRM nut initially did not. Maybe, as Swarfcut said, slight difference in thread size or plating or something was enough to make a differnce. I did not want to force the SRM nut on as i could haved damaged the crankshaft and also looking up thread charts found 3 threads of 3/4" x 20 tpi, one Cycle thread, one UNF and one Whitworth, just to confuse things further. I know there are a few UNF and some BSF threads that are the same tpi as BSCy or CEI. The new original type nut only cost £12, better than stripping the crankshaft thread, and I can always try selling it on ebay for slightly less.