Hex Key (Allen Key)

• Tightening and releasing socket head screws
• Working in a recessed counterbore where a wrench will not reach

Matched on the drive form recorded against each standard, not on the tool's own claims — so this list changes when the fastener data changes, not when this paragraph does.

• Nominal size is the width across flats of the hexagon, in millimetres for metric screws and in inches for unified ones. An undersized key is the classic cause of a chewed socket.
• Long-arm keys reach a recessed socket and give leverage; a ball-end reaches an angled approach at the cost of torque capacity — ball ends strip sockets under load.
• For anything with a specified torque, use a hex bit in a torque wrench. A key gives no indication of what you applied.

• A worn key damages screws. Once the corners of the hexagon round off, the key starts spreading the socket instead of turning it, and the screw is the expensive part.
• Stainless socket screws gall when driven with a stainless key. A trace of anti-seize on the key, and lubrication on the thread, is what stops a joint seizing before it is tight.