Twin discs on the 250n?

So the first thing is that your CB250N/NA wheel may already be able to take two discs (where the CB250NB may not), just take the plate cover of the starboard side of the wheel, and the hub should have a further 6 tapped holes to secure the right hand disc.

CB400N 240mm discs
CB250N ~256mm discs.

There is a CB750/CB650 that had a Calipers very similar to your left caliper. So one more SD disc and a quite rare additional Tokico Caliper.

The CB400N/NA/NB uses NISSIN Single pot calipers on hangers to suit the 240mm discs, they can be used with 256mm discs with matching CB750/CB900F caliper hangers.


So just to complicated things the Single disc 250 set-up used a 14mm Mcyl bore. The two disc set-ups use 5/8" bore mcyl.  The 14mm would operate the twin disc set-up but with more lever travel.

The 14mm Mcyls are more rare than the 5/8" master cylinders, is your 14mm Mcyl in good condition?

The twin disc set-up adds a bit of weight, and you would need a new right hand fork case to mount the additional caliper.

There is a twin pot configuration, that may be simple to do, if your master cylinder is good;-
XBR 500  caliper that comes complete with it's mounting bracket, combined with a disc from a CM400C, CB750F2, CB900F2B/C(?) the disc that is the same diameter as the superdream disc looks similar, but has the lightening slots due to the narrower pad contact area. This was a Standard on the CB400T (1980/81??) in America, and then the CB450T as well (twin 30mm pistons). It was also the standard set-up on the CB250NDD (sold in Switzerland only). The XBR 500 has the correct mounting bracket, as a lot of the models at that time had that protrack anti-dive, so no left brkt that is suitable for the SD. which many calipers would fit. The  XBR 500 has the + pattern on each pot, rather than HONDA on the side of the caliper. 

The XBR disc does not fit the SD





An example of the disc (should get CB900F2B / cx500E more locally)

www.ebay.de/itm/1981-Honda-CM400C-Custom-FRONT-BRAKE-DISC-ROTOR-OEM/162802169961?epid=1411807873&hash=item25e7c3f869:g:pVMAAOSwdG9aMZWB

My master cylinder needs replacing tbh. It’s in bad shape. I’m leaning towards a more modern metal master cylinder. Not original but I do prefer the look. I assumed I would need a new master cylinder and of course a different brake line. Also I assumed I’d need 400 rim.

You explained it all so well and now I just don’t think it’s worth doing. Unless I was very bored with a lot of time on my hands. I don’t have much time unfortunately. I have a lot of other things to do to the bike atm as-well . I’ll leave this one of the list for now.

Thanks so much. I really appreciate all your effort!!