Will this adapter do a 203mm rotor on a 180mm rear PM frame?

Seems likely but some manufacturers have different adapters for each end to try and make sure the forces are going in the right direction through the mounts (apparently). Not sure how much to believe this but I’m not an engineer.

theoretically yes. you’ll have to try it to see if it does fit. if its not a perfect fit then you might need to add some washers between the caliper and adapter to move the caliper up into the correct position on the rotor

Thanks both 🙂

I would guess not, because a 180mm post mount is a 180mm post mount whether its front or rear (IS used to be 20mm smaller on the rear), so that adaptor is actually +43mm (or thereabouts).

What you need is a +23mm addapter or a +20 and some shim washers, hope do 0.8mm ones, so 2x of those under each bolt is 1.6mm (which is close enough to half of 3mm).

The addapter you want will look something like this. To be safe just get the same manufacturer as the brakes, it’s more likley to work.