I've been using a 530 o-ring chain for a long time without any clearance problem. I've known a few other people doing the same. You need to be careful shimming the primary and not over tightening the center-nut, which can warp the inner cover. MkIII's don't seem to be bothered by 530 o-ring chains.
Been using Regina's medium duty ORS2 which is supposed to be more narrow, but it seems to be able to handle the brute horsepower of my 850 up to now. The rivet masterlink may be shorter than the clip type. I happen to have a new chain coming today, I'll measure them both.
snip
hmm, interesting! would be much less expense and hassle than converting to 520 in order to run an O ring chain. I am currently using a fairly wide RK 530 without problems (with the master link clip on the inside) so that just might work
FWIW some of the Regina specs (including the pin length of 24.5mm) are at
http://www.reginachain.it/eng/products/production_range/scheda_or.shtml
so are you using the rivet type master link?? looks like the Regina chain comes w/ both rivet and clip types