What Is a Hex Wrench?

Hex wrenches have a hexagonal cross-section and are often L-shaped.

A hex wrench is a mechanic's tool used for loosening and tightening fasteners. The wrench is a bar of metal formed with a hexagonal (six-sided) cross-section of a precise size. The end of the wrench fits into a hexagonal hole on the top of a hex screw.

Shape

Hex wrenches, also called Allen wrenches or hex keys, are often L-shaped to provide greater leverage when loosening or tightening screws. Because both ends are hexagons of the same size, the wrench can be used in either orientation. Long, straight wrenches with a T-shaped handle on the end are also available. A variation of either design comes with a "ball end," which allows insertion into the screw at an angle, normally difficult with straight-end designs.

Sizes

Hex wrenches are available in both metric and imperial, or SAE, units. Metric wrenches are sized in millimeters (mm), generally ranging from 1.5mm to 10 or 12mm. Imperial wrenches are sized in 32nds of an inch, ranging from 1/16 inch up to approximately 1/2 inch; larger sizes of either format are available. Groups of metric or imperial wrenches are often sold as a set.

Use

Hex wrench sizes are precisely manufactured to specific sizes. A wrench that is too large will not fit into the hole in the screw cap. Use of an undersized wrench risks damaging the screw head, the wrench or both.

References

• Tooling U: What Is the Definition of a Hex Key
• Bondhus Tools: Premiere Ball End Tools
• Aubuchon Hardware: Allen Wrench Sets