Give you great grip when tightening/loosening the head to change over drill bits.Good quality hardened and tempered steel is used for the bit clamps at the head and are toothed to make sure you have a tight grip on your drill bit. For use on rotary and impact power drills, cordless drills and drill drivers. Keyless Design - Tool-free accessory allows you to quickly and easily lock and release. 13mm ( 1/2” ) X 20 UNF