I am looking for the bolt that holds the caliper to the swingarm. #12 in this diagram.
http://www.boats.net/parts/search/Yamaha/Motorcycle/1979/XS400F/REAR DISC BRAKE-CALIPER/parts.html
http://www.boats.net/parts/search/Yamaha/Motorcycle/1979/XS400F/REAR DISC BRAKE-CALIPER/parts.html