Skyjacker 2011-2017 GMC Sierra 3500 HD 4 Wheel Drive Suspension Block and U-Bolt KitSkyjacker utilizes Blocks and U Bolts in its Kits to provide rear lift. The Skyjacker U Bolts are e coated for corrosion resistance. These U Bolts are offered in various shapes and sizes such as square, round, and semi round in lengths ranging from 1. 75 19 inches. The Blocks used in Skyjacker Kits are Ductile iron with fixed center pins. They are also tapered to correct pinion angle and prevent driveline vibrations. Available in 1. 25, 2, 3. 5, 4. 5,