Current bike has 180mm rear post mount, ie. 180 is the smallest rotor it will take, which it does WITHOUT adapter. If I want to run a 203mm rotor I assume I need a post to post adapter normally used to run a 180mm rotor in place of a 160mm rotor,

ie. something like this
http://www.uberbikecomponents.com/view-product/Uberbike-Front-Post-to-Post-180mm-Disc-Brake-Mount-adapter-Colour-Options

rather than a 203mm adapter, like this

http://www.uberbikecomponents.com/view-product/Uberbike-Front-Post-to-Post-203mm-Disc-Brake-Mount-adapter-Colour-Options

since I am effectively only going up one size???

Is this correct? Or is there another type of adapter I am missing.

What I did find with some IS to post adapters (shimano in particular) was that they were sold as front or rear not both. I am not sure if they were actually different shapes. My suspicion is just labelled differently to reflect that a rear 160 adapter was in effect a front 180.

Sorry I don’t think that necessarily helps.

I guess what I am asking is whether a post to post 160-180mm adapter is essentially the same as a post to post 180-200mm adapter. Pretty sure it doesn’t matter whether it is front or rear.

Thinking about it I am pretty sure my Pikes on the front will take 180mm minimum rotor and I had to use a 160-180 adapter on them to fit the 200mm disk. So I think that answers my own question. 🙂

yeah post to post should be either just front or back and think of them as a +20mm.

The shimano IS to post are actually different front and rear as the holes on one of them as slightly off centre. You can use a front IS to 160mm adapter to fit a 140mm rotor to the rear of an IS frame but will need some shims to space it in a bit. Most generic IS to post don’t have this problem.

Thanks Andy for confirmation, and useful info on the IS mount.