I have a bicycle brake disk that I wish to mount on a (bicycle) hub. The
disk is RH threaded and is meant to mount on the left side of the hub, so
when the brake is applied it screws down onto a shoulder on the hub. My
application means I need to mount this disk on the right side.

So what I think I'll do is make a shouldered collar threaded RH on the
outside, and LH on the inside, and cut LH threads on the hub. The disk will
run up to the shoulder on the collar and the collar will run down to the
shoulder on the hub.

How thin can I make the collar? - there's a limit to how far I can reduce
the hub diameter. The disk is threaded 1.375 x 24TPI; I'll probably make
the collar from either brass or mild steel; and the hub is some kind of
aluminium alloy.