Avid's Rear 170mm Post Bracket Hardware Kit has everything you need to mount a 170mm rotor (sold separately) to the back of your bike. Kits include mounting bolts and a 30mm post mount spacer for rear brake installation.
With Avid's CPS (Caliper Positioning System) hardware, getting a perfectly aligned rotor has never been easier. Just (Step 1) loosen the caliper bolts, (Step 2) squeeze the brake lever, and (Step 3) re-tighten the caliper bolts. Perfect caliper alignment - just like that.