Allen wrench

Allen wrench. Allen wrench is the tool used to screw / unscrew screws, which have an internal hex head. Compared to a philips screw, it resists higher torques.


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  • 1 Description
  • 2 Features
  • 3 standard sizes of hex keys
  • 4 Sources


Originally Allen was a registered trademark of the Allen Manufacturing Company in Hartford, Connecticut in 1943 .


  • Simple, small and light design.
  • Screw contact surfaces (internal) are protected from external damage.
  • It can be used with screwdrivers or headless wrenches (using a fixed wrench for example).

The screw can be inserted into its slot using the screwdriver directly (they fit perfectly).

There are six contact surfaces between the screw and the screwdriver. The pair is spread throughout the key.

Can be used with very small screws.

The manufacture of Allen keys is very simple, so many times one is included along with the screws.

Standard sizes of hex wrenches

Hexagonal wrenches are named for their distances between faces, the measurements standardized in millimeters by ISO 2936: 2001 are as follows: 0.7, 0.9, 1.0, 1.25, 1.3, 1.5, 2 to 6 in 0.5mm increments, 7 to 22 in 1mm increments, followed by 24, 25, 27, 30, 32, 36, 42, and 46mm.

Metric hex wrenches are normally called with an “M” followed by the size in millimeters, for example “M8”.

Apart from the metric normalization in millimeters, which is generally used in Europe , there is the normalization in inches, commonly used in the USA. USA

Using a wrench on a screw with a larger housing can cause damage to the tool or screw by reducing the contact surface to only the corners. The above situation usually occurs when the available wrench set is metric and the screws are in inches or vice versa.

Some wrenches have a hexagonal pseudo sphere at the tip that allows adjusting / loosening the screws with the wrench in the off-axis position. This feature weakens the wrench and decreases the contact between the wrench and the screw, increasing the chances of damaging both.


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