Specialized Chisel FS frame - 160mm to 180mm rear brake

Looks like you need a proprietary 180mm adaptor from Specialized.  Or just use a 160mm disc

A little google suggests others have simply bolted a Shimano +20mm adaptor to the one that comes with the frame.  You'd better check clearance for the caliper and bolt lengths if you go that route

Thanks, currently set up with a 20mm Shimano adaptor, just not aligning right. Will speak with Specialized and see if I can get their 180mm mount.

Any +20mm adaptor should work fine in theory; has the brake mount been faced? Pictures showing the issue might help.

Each component has tolerances... so if you add an adaptor on top of an adaptor, you could potentially end up out of tolerance.

If you can get a dedicated 180mm adaptor, it would be better IMO. Here's a pic of the OEM 160mm adaptor for reference.

I know in Shimano land at least, there are different disc+caliper standards (ie some calipers are only designed to work with certain discs), so you can double check your caliper + rotor combo just in case.

Specialized tech team said there is no 180mm adaptor for the bike. They couldn’t recommend any mount, so back to playing around with washers and longer bolts again. If all else fails then I need to drop to a 160mm rotor 

Just spoke to someone in the know. You need an adaptor on an adaptor and some spacer washers. Install standard Specialized mount adapter, add 20mm Shimano adapter and add several spacers until it aligns correctly. Not a clean solution, but appears to be the only solution. 

I wouldn't stack two +20mm mounts on top of each other but when the stock brake mount is weird like this and not a standard post mount I would. My focus is similar that the stock mount needs another one on top. 

IMO I've always had the best success with hope mounts as they are machined. Stif have them 50% off at the moment and sell the Hope Disc Caliper Shims in different sizes as well.