Hey Steptoe, I had the same problem but did a slightly different mod. I also had time on my hands and you have a spare bolt. I welded an M12 nut directly onto the existing bolt head, after drilling the nut out so that it fitted all the way down onto the flange. (I removed the 'o' ring spring etc before welding). After removing any weld spatter and giving the bolt a clean up I got it cadmium plated, re-assembled everything 'o' ring etc and refitted to bike. I now use a 17 mm socket to remove. It's crazy that the Japenese use such a small bolt head for such a large bolt, it will round off sooner or later for sure.
