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Definition: L-shaped tool used for the installation and removal of fasteners with a hexagonal (6 side) head; consist of a single piece of material, typically metal, formed into a right angle

What is an allen key/ allen wrench/ hex key?

Allen keys are formed from a single piece of hard hexagonal (6 sided) steel rod, having blunt ends that fit snugly into similarly-shaped screw sockets.

The rods are bent to 90º, forming two arms of unequal length resembling an “L”. The tool is usually held and twisted by its long arm, creating a relatively large torque at the tip of the short arm. It can also be held by its short arm to access screws in difficult-to-reach locations and to turn screws faster at the expense of torque.

Hex keys are designated with a socket size and are manufactured with tight tolerances. As such, they are commonly sold in kits that include a variety of sizes.

Why is it called an Allen key?

Originally named Allen Manufacturing Company, the business produced hexagonal set screws and wrenches to fasten them. The terms “Allen wrench” (American English) and “Allen key” (British English) are derived from the Allen brand name and refer to the generic product category “hex keys“.

W.G. Allen filed the first related patent in 1909 for its recessed hex-driven safety screws, a safety improvement over fasteners which protruded from machinery.

Types of Allen keys

L-shaped: feature a hexagonal profile bent in the shape of an “L”. Either end can be inserted into the fastener socket depending on the clearance. Typically made of alloy steel and available in two different lengths – short and long.

The length is the dimensional measurement of the length of the long arm.

T-shaped: use a handle formed from a bend in the metal stock that leads to a single drive key centered across the handle width. Provides for a more comfortable action in repetitive use and allows for greater torque to be applied than with the traditional L-shaped Allen key.

The length is the dimensional measurement of the length of the long arm.

Ball-end: feature a hexagon-shaped ball at the driving end of the tool. These types of Allen wrenches are available in both L-shaped and T-shaped designs.

The use of the ball end design on the driver provides for the ability to tighten or loosen fasteners when the tool needs to be held at an angle with respect to the fastener (i.e. other than the driver at 90 degrees relative to the plane of the fastener head).

This is valuable for applications where the working clearances are such that a head-on use of the tool is not possible or where the direct engagement would not leave sufficient room to rotate the tool to apply torque.

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Key features of the Allen key

Some features of hex keys are:

• The tool is simple, small and light.
• The contact surfaces of the screw or bolt are protected from external damage.
• There are six contact surfaces between bolt and driver.
• Very small bolt heads can be accommodated.
• The tool allows the use of headless and recessed-head screws.
• The screw can be held by the key while it is inserted into its hole.
• The torque applied to the screw is constrained by the length and thickness of the key.
• The tool is cheap to manufacture, so it can be included with products requiring end-user assembly.
• Either end of the tool can be used to take advantage of reach or torque.
• The tool can be reconditioned by grinding the worn-out end.

How to choose the correct Allen key size

The important parameter that establishes the size of the Allen™ wrench or Allen™ key is the width across flat edges, which is the distance between any two of the faces of the tool that are parallel to each other.

Allen keys are available as individual keys of a given size but are also sold in sets that contain a range of standard sizes. Allen keys are identified as being either metric-sized keys in millimeter sizes or SAE (Imperial) sized keys in fractions of an inch or decimal equivalents.

Within a given size, the other options that the buyer needs to consider are the handle style, the material, the finish, and the length of the tool (usually designated as short and long).