Socket head screw, also known as socket head screw or Allen bolt, is a screw with a hexagonal inner hole in the head, which can be tightened or loosened only after being inserted into the inner hole with a hexagon square. The most common use of hexagon screws is cylindrical head screws, head diameter about 1.5 times the main diameter of the thread, other head types include semi round head screws and suitable for conical screw hole flat head screws. Countersunk hole design allows the screw head to rotate without being exposed to the surface of the fixed object, so it is often used in small surfaces where traditional spenders are inconvenient to use.

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