I have a 2008 Iron 500 ES. I grabbed a couple nuts last night - but both seemed to be the wrong size. For the hell of it I grabbed both a metric and a standard size - what looked to be right in size - but alas - neither wanted to thread in - the problem is when I put a nut in line with the bolt - they barely fit in the space - they keep hitting the clutch case bottom... if I get a slightly larger one I fear it wont fit in that space at all... dont know until I try it though. Plus talk about annoying - I cant get anything in that space unless the center stand is up - making this a real pain on a garage floor with no lift to elevate the bike. Needless to say I was a bit knackered after the struggle having my son hold the bike up while I toiled in angry fumbling with the nuts. haha. It just occured to me --- when you say british thread - is that basically not "righty tighty" like US threading - but "lefty tighty?" threading it on turning counter-clockwise? I'm at work so I cant do anything with it now. I didnt even think to try that last night - DERP!
Possibly #27 in this illustration?http://www.hitchcocksmotorcycles.com/partsbooks/pages/1638/Frame_Assembly