Are the shimano post to post adapters the same front and rear? They seem to be sold as for the front, and the only rear specific adapters are IS to Post.

They’re fine on the rear, I used one on my last frame with no problems

I think most bikes are designed to run 140 rear and 160 front disc without an adapter . A 180 front adapter will therefore only be raising the brake size by 20mm so if used on the rear would space the brake to 160mm .

Just bought one for a mate it was a front post to post 180,its been fitted to the back with no problems to a giant anthem

What RN said is true for most frames and forks. Some confusingly will take a bigger disc without adaptors. It’s easier to think of them as “+ mm” adaptors IMO so a front 180 is +20mm.

thanks. Both my bikes are post mount on the rear designed for a 160 rotor, so in alps prep I’m going up a size on the rotor. Looks like the normal 180 post to post adapter will work fine then (robbed from the front as that’ll up-sizing to 203).