I have a fork that as standard accepts a 180mm rotor on a post mount. I want to fit a 203mm rotor and cannot for the life of me work out what mount I need to fit it?

What fork? Some, like the x fusion slants 2013 don’t require one.

Fox 36 2015. I can bang a 180mm rotor on it and it will be fine but I want the 203mm on. It does need a adapter, I just don’t know what!

Hummm, then what brakes? Maybe you just need a Shimano or whatever 203 post mount adapter?

Surely just a 20mm Post to Post adapter or am I missing something? So since your fork mounts fit 180mm rotors directly then a 180mm Post-Post mount adapter (i.e. it the adapter is normally 160 + 20mm = 180mm hence the name, but for your fork will be 180 + 20mm)

I tried the 180mm adapter but it does not work as the disk is 203mm not 200mm.

Most adapters come with some washers. Just run 1.5mm worth of washers underneath the adapter to raise it slightly.

You’d need a 183 post to post adapter if your fork is a 180post mount as standard as you need 23mm lift to get from 180 to 203 the same as post mount fork going 160 to 183..
Hope do em..
Like this the 183 version not the 203 one as that’ll lift it 43mm not 23mm as they’re for 160mm post mount forks as standard

^Thanks I have just ordered one.

http://www.chainreactioncycles.com/shimano-mount-adaptor-front-post-to-post/rp-prod13747

Am I missing something? Surely you just need the standard 203mm post to post as per linky ^

You’re missing the fact that the standard post mount 203 adapter is for a 160 post mount fork and the OP has a 180 post mount fork