Special screw with left hand screw thread to lock a chuck to the motor shaft. This screw performs a very important role. If one uses a drill in reverse rotation, the chuck will come off. This screw locks the chuck in place. For your own safety, always install the screw when replacing a chuck.

Chuck removal:
To remove the chuck, loosen the screw by turning it in the clockwise direction. The chuck loosens in the normal direction i.e. turning anti- clockwise

Chuck installation:
The chuck will screw on in the normal clock wise direction and the screw will fasten in the anti-clockwise direction.