What Is a Socket Screw?

Lori Kilchermann

A socket screw, or Allen head screw or bolt, as it is otherwise known, is a type of fastener that uses a hexagonal wrench often referred to as an Allen wrench or Allen key to tighten and remove. This fastener is used in a myriad of applications where a strong bolt or screw is called for. Requiring less room than the common hexagonal nut and bolt requires for placing a wrench on the fastener, the socket screw is often run in tight tolerance applications where space is at a premium. Often used in the building of machines, high-performance applications and aircraft, the socket screw is a very high-strength fastener that is commonly made from the highest grade of steel and hardened aluminum to ensure a quality fastener is created for every application.

Allen key set, like those used with socket screws.
Allen key set, like those used with socket screws.

The most common type of socket screw is manufactured from high-strength tool steel, however, these fasteners are also made from many other types of steel and alloy metals. Stainless steel, aluminum and titanium are also used to manufacture the socket screw. Made famous for the very high breaking point of the fastener, the socket screw is able to be used in extreme duty applications where even the highest grade of common fastener could fail. Manufactured typically in a very fine thread configuration, the fasteners are also available in coarse thread versions.

A socket screw.
A socket screw.

The fine thread pitch of the fastener is so adept at holding tight when under torque that locking washers and nuts are seldom used under any circumstance. The flat surface found on the underside of the fastener's head is such that it evenly applies the load over the entire surface of the screw head without the assistance of a flat washer. Many instances of flat washer use are based on the type of metal that the fastener is being used in, with a softer metal requiring a flat washer. This is done to avoid the tendency of the socket screw to dig into the surface of the metal it is securing.

The head of the socket screw is smaller than a comparably-sized hex nut or bolt, so it requires less physical space to occupy the fastener. The use of an Allen wrench or Allen head socket to install and remove the fastener allows the user a choice in tool depending on the different clearance issues. When properly torqued, the telltale sound of a socket screw being broken loose is an audible snap or snapping sound. The snap is indicative of a quality fastener providing substantial holding force.

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