The Hitachi HiKOKI 311959 M6 x 23 Special Screw is a left-hand threaded replacement chuck-retaining screw for a wide range of compatible Hitachi and HiKOKI drills and driver drills. It secures the drill chuck to the machine and uses a left-hand thread to resist loosening during normal operation, making it suitable for replacing a damaged, stripped, worn or missing original screw.