The Speedco M2 Pro Disc Brake Mounting Kit is machined out of a black of 6061 aluminum and will work only with a 120mm rotor. Don’t end up on the naughty list by trying to use a 140mm rotor. Sold as a kit with the brake mount arm, three axle blocks, and two chain tension screw-nut combos. Available in three sizes, so measure twice, order once.