A hex key (also called an Allen key or Allen wrench) is a hexagonal bar used for turning screws. Hex keys are sized from the distance from one flat to the opposite flat, and come in a variety of sizes for adjusting different sized fasteners.
Typically a hex key has a long part and a short part; the longer part can be used for giving extra reach, while the short part can be used when extra torque is required. Hex keys can also come in ball ended varieties, where the longer part ends in a hexagonal ball, making it easier to turn screws (especially in arkward places) but giving lower torque before the tool will cam out.
Hex drivers are a variety of hex keys resembling screw drivers.
Over time hex keys wear, and will cam out of the fastener too easily. Small hex keys can easily be returned to as-new condition by cutting off the worn part with a metal cutting disk.
If the head of a cap head bolt becomes damaged, they can often be removed by cutting a slot in the head with a metal cutting disk and using a flat-blade screwdriver to remove the bolt.
- Hex key on Wikipedia.
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