Bought mine last November (2001 600, 14000 on the clock) and checked it due to reading about it on here and it was loose. Bought the 12mm one , was less than 3 quid if I remember right, including the lock washer, so stupid not to do it. Can also recommend the windy gun for undoing front sprocket nuts that won't come undone, had same problem twice with RD250F and a 350 powervalve, really struggled until someone else suggested a windy gun, borrowed a cordless one from work and it came off instantly, ended up buying one for my compressor off ebay for about 22 quid (including some sockets) and it has helped out one or two friends now.
