sent this as a PM to someone but then realized it is probably valuable information to this thread:
To do the twin caliper on the rear you need the caliper bracket (that goes around the axle), the caliper and the banjo bolts. I recommend the torque arm too but there is a VFR inteceptor aluminum arm that some of the DOHC guys like to swap out for because it looks trick (I forget the year VFR - 1984 I think, it is the one with a clevis at both ends). Again I recommend finding one off a 1981-82 750F but I will post the part number and bikes that use it below. Unfortunatly it is a much smaller number of bikes. Honda has two diferent part numbers for the bracket itself - I assume this one: 43110-445-632 is a revision number in the part but I don't know what the changes are - it shouldn't make a difference. I can tell you that Honda used the same rear axle diameter all the way to the first generation cbr 1000s. I have a 1986 VFR 4.5 inch rear wheel on the back of my 1978 cb750F with the VFR caliper and bracket (and a GS1100 swingarm) and I use the stock 1978 cb750F axle (and most of the spacers). Just to be on the safe side - I would be ready to shell out for rear axle spacers off the same bike. The DOHC uses an offset swingarm and thus has different length spacers. IIRC the bracket is the correct thickness to replace the stock 77-78 one but just in case be prepared to make your own or play mix and match with the SOHC and DOHC rear axle spacers.
Assemblies where 43100-445-634 or 43110-445-632 is used
1981 CB750F A - REAR BRAKE CALIPER
1981 CB900F A - REAR BRAKE CALIPER
1981 CBX A - REAR BRAKE CALIPER
1982 CB750F A - REAR BRAKE CALIPER
1982 CB900F A - REAR BRAKE CALIPER
1982 CBX A - REAR BRAKE CALIPER
It is a common misconception that the rear caliper is the same as the fronts (this might be true for the single puck stock 77-78 calipers). However here is the parts cross over for the rear caliper:
Assemblies where 43101-MA3-006 is used
1981 CB750F A - REAR BRAKE CALIPER
1981 CB900F A - REAR BRAKE CALIPER
1981 CBX A - REAR BRAKE CALIPER
1982 CB750F A - REAR BRAKE CALIPER
1982 CB900F A - REAR BRAKE CALIPER
1982 CBX A - REAR BRAKE CALIPER
1982 FT500 A - REAR BRAKE CALIPER
1983 CB1100F A - REAR BRAKE CALIPER
1983 FT500 A - REAR BRAKE CALIPER
In all honesty though, I have only done 1 rear conversion. I did it because I wanted the calipers to match on the bike, and the rear had gone bad anyway and service parts for the duals are surprisingly cheaper than the single pucks. I didn't feel it added anything to the riding experience, the rear is so undertaxed anyway that the single puck was more than up to the job. It is really so they match and look cool, esp when you paint them gold and then letter the raised honda badge in red (or paint them red and letter honda in white like brembos). If they are just going to be black, it won't stand out enough for people to notice the single puck in the rear and most bikes have different rear calipers than the fronts anyway.
