Flat mount is a cleaner brake standard than post mount, but it introduces one wrinkle: the caliper position is fixed relative to the frame, so changing rotor sizes requires an adapter. That's what the Shimano SM-MA R160D/D handles. It bolts between a rear flat mount caliper and a flat mount frame originally designed for a 140mm rotor, adding 20mm of offset so you can run a 160mm rotor instead. It's the standard fix when a customer wants more heat capacity and stopping power out back for loaded gravel days, tandem duty, or big descents.

The adapter itself is machined aluminum, and Shimano ships it with the specific hardware the flat mount interface needs — the tapered-head caliper mounting bolt plus the bolt mounting pin that keeps everything from backing out under vibration. Torque spec on the caliper mounting bolts is 6 to 8 Nm, and Shimano's install sheet also calls out a 13mm protrusion length for the frame-side mounting bolt, which is worth checking against your frame's mount thickness before final assembly. If the protrusion falls outside that spec, you'll need a different bolt length from Shimano's flat mount hardware selection.

One thing worth flagging: this is the rear-specific adapter. Front flat mount conversions use a different part. And while Shimano designed this around their own calipers, the flat mount standard itself is shared across the industry, so it works with SRAM and Campagnolo flat mount calipers running a 160mm rear rotor as well. If you're moving from 140 to 160 out back, this is the piece that makes it happen without any frame modifications.