The Allen wrench used to be a common tool only for those seeking to repair bicycles or motorcycle engines. Also called a hex key or hex wrench based on its hexagon-shaped head, the Allen wrench became popular a few decades ago as the do-it-yourself tool to unclog under cabinet residential garbage disposals. While many did not know what it was called, it was commonly used to fit into a small hole on the top of the garbage disposal unit and free the mechanism with a quick turn. More recently, the Allen wrench became a common tool to assemble Swedish-made furniture at home.
The Allen wrench has a hexagonal cross section that fits screw heads with six-sided recesses. The tool is unique in that it fits into a receptacle to turn it rather than around a screw or bolt like a regular wrench. This makes it the only tool that can access fasteners that have no protruding heads and are flush with the object being assembled or repaired. The surfaces of the screws and bolts are protected from damage because they are recessed. Consequently, motorcycle engine covers, for example, can be easily mounted flush to the engine case without the heads of the bolts sticking out.
Other features that make the Allen wrench popular are its small size and light weight. It is also unique in that it can reach very small and buried screws with ease. The ends of an Allen wrench are conveniently interchangeable. Torque is limited by the wrench's length and thickness.
Allen wrenches are available in metric and fractional inch sizes. The three most popular sizes for automotive and motorcycle repair are 5/32 inch, 3/16 inch, and 1/4 inch, and in metric sizes, 4 mm, 5 mm, and 6 mm are favored. Other preferred Society of Automobile Engineers (SAE) sizes are 3/32 inch, 7/6 inch, 1/8 inch, 9/64 inch, and 7/32 inch. The sizes used to assemble furniture vary depending on the piece of furniture. Allen wrenches for furniture assembly are normally included in the box with the product because they are so inexpensive to manufacture.
The most common complaint about the Allen wrench is that it is easily misplaced or lost. Since its length is less than half of that of even a socket wrench, and its circumference is less than that of a standard pen or pencil, it often gets tossed out with the box the furniture came in or inadvertently gets swept up with debris in the garage. The most popular way to keep track of different size Allen wrenches is to keep them separate from other tools in a special container.
Also known by the names zeta, Unbrako, or Inbus, the invention of the Allen wrench is often mistakenly attributed to someone named Allen. History indicates the first tool of this type was developed in the early 20th century by the Standard Pressed Steel Company. The trademark for the design was taken out in 1943 by the Allen Manufacturing Company.