The terminal bolt on the 620 starter motor has sheared off, then trying to remove the under nut has left the post rotating. To get at the long bolts on the starter I needed to buy a cranked 8mm ring spanner to get around the oil cooler hookups. Turned out the post had broken away at the spot weld to the brush wire, and also the plastic insulation had broken up as the bolt twisted. Found a local bike parts manufacturer,
http://www.electrexworld.co.uk/acata..._Kits_327.html, that do a unit suitable for all duc twins from 1986, for £25 posted. It comes with all the parts including the insulation. They also have complete starter motors for around £90.
Is there a forum section for matching duc parts?