How it connects
The male hexagonal key slides into the matching recess; torque is transmitted through the six flats. No seal or retention; it is a temporary tool engagement.
Male and female halves
Female: the hex recess in the part (this connection). Male: the hex key or hex bit, sized by width across flats.
Usually connects to
- Matching-AF hex/Allen key, including L-keys
- Hex driver bit or ratchet hex-bit socket
Common sizes
- Sized across flats: metric about 0.7–27 mm, commonly 1.5, 2, 2.5, 3, 4, 5, 6, 8 and 10 mm.
- Inch sizes run about 0.028–1 in, commonly 1/16, 5/64, 3/32, 1/8, 5/32, 3/16, 1/4, 5/16 and 3/8 in.
- A leveling-foot example is about 4 mm across flats.
Where it is used
Socket-head/set screwsFurniture fittings/assemblyMachinery construction and leveling feetBicyclesAdjustable spring-hinge tensioning
Variations
- Metric or inch/SAE AF size
- Center-pin tamper-resistant hex
- Ball-end-compatible socket
How to recognise it
A six-sided, flat-walled recess usually in the end face of a screw head or stem, often with a conical bottom from drilling. Size is the distance across opposite flats. Often supplied with a matching L-shaped hex key.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| Torx (hexalobular) socket | Torx has six rounded lobes like a star; hex has six straight flats and sharp corners. |
| external hex (hex head / hex nut) | External hex is a male six-sided head turned with a wrench or socket; the hex-key socket is a recess. An external hex is a male hex profile turned with a wrench, not a recess. |
| metric vs inch hex of near size | Measure across flats; e.g. 5/32 in (3.97 mm) vs 4 mm fit loosely and strip easily. |
| Square (Robertson) socket | Square has four flats, hex has six. |
| External hex (wrench flats/hex head) | External hex is a male hex profile turned by a wrench; internal hex is a recess turned by a key. |
Also called
Allen socketinternal hex drivehexagon socketAllen driveinbus (DE)hex socket drivehex recess
Good to know
This is a tool-drive interface. A hex key can turn the stem of a leveling foot for adjustment; check the across-flats size.
Related connections
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