What Is an Allen Key? Types and Selection Criteria
An Allen key is a hexagonal, usually L-shaped hand wrench used to tighten and loosen bolts and set screws that have a hexagon socket in the head.
An Allen key, also called a hex key, is a hand wrench made from hexagonal steel bar and used to tighten and loosen bolts and set screws that have a hexagon socket in the head. Its most common form is an L shape: the short arm applies force in tight spaces, while the long arm turns quickly and reaches deep-set bolts. It is used in machine and mold assembly and on the socket head bolts of furniture, bicycles and equipment housings.
Types of Allen keys
| Type | Feature | When to choose it |
|---|---|---|
| L-shaped hex (standard or long arm) | Flat ends, both arms usable | General assembly; long arm for reaching deep-set bolts |
| Ball-end | The long arm ends in a ball shape that enters the socket at an angle | Places where the key cannot be held in line with the bolt, fast turning |
| T-handle | The key is fixed to a T-shaped handle | Repetitive work where many bolts are removed and fitted one after another |
| Cassette type | All sizes are kept in a single holder | Tool bags and field work; keys do not go missing |
| Torx | Six-lobe star profile instead of a hexagon | Torx socket screws and bolts |
| SAE (inch) | Sizes are given as inch fractions | Machines and equipment that use inch bolts |
Selection criteria
- Socket type: First determine whether the bolt socket is hexagonal or Torx. The two profiles are not interchangeable.
- Measurement system: Use a metric (mm) set for metric bolts and an SAE set for inch bolts. An inch key of nearly the same size may enter a metric socket, but it leaves play and strips the socket.
- Size range: The set should cover the bolts you work with most often. Socket head cap screws (ISO 4762) need a 2.5 mm key for M3, 3 mm for M4, 4 mm for M5, 5 mm for M6, 6 mm for M8, 8 mm for M10 and 10 mm for M12. Button head and countersunk screws take a smaller key for the same thread diameter.
- Length and tip shape: Choose long arm for deep-set bolts, ball-end for angled access and T-handle for repetitive work.
- Material: The key must not twist in the socket or round off at the corners. Where the catalog gives a material for the İzeltaş sets, it is chrome vanadium.
Common mistakes
- Final tightening with the ball end: The ball end contacts the socket over a smaller area. Use the flat end to break a tight bolt loose and for final tightening.
- Not seating the key fully: A key that is only half inserted strips the corners of the socket. Clean out dirt and paint and push the key all the way in.
- Extending the arm with a pipe: The key twists or breaks and the bolt socket is damaged.
Related standard
- ISO 2936: Specifies the dimensions, test method, minimum hardness values and marking of hexagon socket screw keys.
Allen keys in the Welibaba catalog
The Allen Keys category has 7 active products, all of them İzeltaş sets:
| Set | Socket and size | Stock code |
|---|---|---|
| Long arm, 10 pieces | Hex, metric 2–10 mm | `4903003110` |
| Ball-end long arm, 9 pieces | Hex, metric 1.5–10 mm | `4906003109` |
| Cassette type, 7 pieces | Hex, metric 2.5–10 mm | `4912003107` |
| T-handle, 8 pieces | Hex | `4920008108` |
| Torx, 8 pieces | Torx | `4910007108` |
| SAE, 10 pieces | Hex, inch | `4901003110` |
| SAE, 12 pieces | Hex, inch | `4901003112` |
For hexagon head bolts and nuts, see Wrench Sets; for working with sockets, see Socket and Bit Sets.
Frequently asked questions
Are an Allen key, a hex key and an Inbus key the same thing?
Yes. They are different names for the hexagonal key that turns hexagon socket bolts.
What size Allen key does an M8 bolt need?
An M8 socket head cap screw (ISO 4762) needs a 6 mm key. M8 button head and countersunk screws take a smaller size (5 mm); if you are unsure, check the bolt's standard table.
What is a ball-end Allen key for?
The ball end enters the socket even when the key cannot be held in line with the bolt, allowing fast turning in obstructed and tight places. Because the tip carries less force, loosen a tight bolt and do the final tightening with the flat end.
Can an SAE Allen key set be used on metric bolts?
No. An SAE set is for inch bolts. An inch key of nearly the same size leaves play in a metric socket and strips it; choose a metric set for metric bolts.