right, this is something i've never really understood. I've got some avid juicys on my bike at the moment and the front has never ever been any good. Looking to get some SLX to replaced. My forks are IS mount and i have a 185mm front disc. Will the SLX post mount brake just fit on the adaptor that my juicy's currently use or will i need a new one?

should fit on probably need to do a bit of trial and error on the disc size though

Not sure if it will fit the current adaptor, as the avids use those conical washers to make setting them up so much more awkward. Going back a few years, shimano brakes came with is-post adaptors, so I wouldn't be surprised if they still come with everything you need in the box. They should come with a new disc as well, so you don't need to worry about sizing.

My SLX's came with IS to Post mounts but my Deores had no mounts at all.

If you are buying just the calipers (ie no rotors) then you will either have to buy a new mount for the 180mm (Shimano don't do a 185mm rotor) or just try to see if your existing mounts and disc work with the SLX caliper which they may well do.

Worst case scenario is you need a new rotor and mount but the avid and shimano 160mm mounts are near identical (just looked as the GF's bike runs Avids) so i reckon you'll probably be ok.

cool, i'll give it a go then. was trying to get away with not buying a new disc tbh. Am only going to be doing one wheel at a time too…

I've put a shimano caliper on an avid 185mm mount with 185mm rotor, found that the wheel would turn, but was grinding on the caliper. Put a 1mm washer between the caliper and mount (1 for each bolt) and hey presto, fully working, grind free brake!
Easy!

just a quick note to say cheers guys. Got the front working with a pair of washers as suggested above.

They are a revelation compared to my juicy 3s. Even had to bleed the front after shortening the hose and that was a fun 10 min job rather than a 30 min struggle as it was with the juicys.