There's 2 types of 180mm post adapters, one adapter uses 2 longer bolts, the other type of adapter uses 4 smaller bolts.





* That 4 bolt adapter isn't made by Shimano anymore but a couple of companies still make that type.

Avid makes post spacers.



* You need to be careful with these as there's 2 versions for each size, one version is for CPS calipers and the other version is for non CPS calipers.
If you use the CPS post spacers on non CPS calipers your pads will overhang the outer edge of the rotor by about 5mm.
Non CPS post spacers won't give enough clearance with a CPS caliper.

https://www.sram.com/sites/default/f...r_mounting.pdf

A 160mm rear PM to IS adapter only comes in one basic type.