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The History of the Allen Wrench

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An Allen wrench or Allen key is a familiar brand name for a hex key or hex head wrench, a tool that drives a screw or bolt called a “socket head cap screw.” This type of screw, with a recessed hexagonal head is common in bicycles, motorcycle engines and, increasingly, build-it-yourself furniture, according to Popular The hex head wrench is also known by the brand name “Unbrako,” sometimes misspelled “Umbrako.”

Screw Manufacturer

According to John Bates of the Traditional Tool Group in an article for, the SPS Company of the United States is thought by some to have invented the socket head cap screw. The company, established around 1911, used the brand name Unbrako, as in “unbreakable,” for the screw and became a world leader in its mass production.

Allen Name

The “Allen screw” name for the socket head cap screw and “Allen key” for the wrench originate from the products of the Allen Manufacturing Company of Hartford, Connecticut. According to Bates, it is widely reported that the company trademarked the name “Allen wrench or key” for its range of hex wrenches in 1943.


Like the Unbrako name, the name of the popular Allen brand influenced terminology regarding the tool. The Allen Manufacturing Company no longer exists, but the Danaher Corporation of Washington, DC. took over the name and continued production of the Allen wrench.

Size Allen Wrench I Need?

An Allen wrench comes in many convenient sizes. The Allen wrench dates back to the mid 1900s. It became the standard name for what had previously been called a hex key. The inch-based set of Allen wrenches is considered standard in the industry. The typical sizes in the Allen wrench European set are: * 2.5 mm * 3 mm * 4 mm * 5 mm * 6 mm * 8 mm * 10 mm If you are working on a project that has shipped from another country, you may need to use a metric set of Allen wrenches. If you truly aren’t sure, you will have to make a good guess. Most manufacturers will publish an Allen key size chart. Hold on to it or post it somewhere in the garage where you can hold up the key or socket screw head to the physical picture. This can help you to determine which size of Allen wrench you need or have on hand. The smaller size of Allen wrench can easily strip a hex bolt or be stripped down itself.

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