If I change my rotor from a 140mm to a 160mm do I need to change anything with the brake caliper mount?
i.e do I need to add spacers or do I need a conversion kit?

I'm running a Fox F120 with an Avid Juicy 3 post mounted (soon to be changed to an ultimate).

You need an adaptor. Check out CRC, Brake Adaptors website. You need to be very careful of the description though – do you need IS to IS +20mm, Post to IS +20mm, IS to Post, Post to Post?

If this doesn't make sense, talk to CRC helpline

I 'thought' standard size for front post mount was 160mm rotor, so 'should' go straight on. No?

My Juicy attached to a post mount fork it runs at 160 mm .. I thought this was standard.
. not sure how that works on your fork is there an adapter between your fork and the brake caliper? if yes take it off = 160 mm post mount. if no you need a +20 mm post to post mount you cannot but a 160 mm post mount due to above

standard front setting is a 160mm rotor, so I'm not sure what you're on about with a 140mm rotor. 140 rotors are just for the rear.

Do you really mean 160 to 180mm ?

In other words, with a 160mm rotor you either just bolt the caliper straight onto your forks (if it's a post mount fork) or use the supplied adaptor (if it's a IS fork).

Hey reggie, I think you're onto something there.

I'm running a 160mm bolted straight in to the forks and want to upgrade to a 180mm. Sorry brain still recovering from twentyfour12.

So I'll need an adaptor then? Probably post to post +20mm like this

yes, but that's for a 185mm disc

to run a 183mm disc

http://www.chainreactioncycles.com/Models.aspx?ModelID=13728

to run a 180mm disc

http://www.chainreactioncycles.com/Models.aspx?ModelID=50751

Ribble has them cheaper though

http://www.ribblecycles.co.uk/sp/mtb-bmx-bike/Disc-Brake-Adapters-Shimano-Disc-Brake-Adapter-Front/SHIMDIBA200