Definition: When a bolt needs to be really tight, sometimes a car will use an allen bolt. An allen bolt has a hexagon-shaped hole in the top. The allen wrench matches that hole with its six-sided steel construction. Allen wrenches usually come in sets, and can be loose or held together on a sort of keychain, as they are often found. You can also buy allen wrench sockets for your ratchet wrench, which can really make things easy when you are taking out a long bolt or you're working in close quarters.
Also Known As: hex key, hex wrench
