Everything You Need To Know About Socket Screws

What is a socket screw?

Socket screws are a variant on traditional screw formats that are designed and manufactured with a hexagonal (hex) internal drive built into the head. As such, they’re intended to be tightened and loosened with an Allen wrench or hex key, as opposed to a traditional screwdriver.

This is really the key feature that sets a socket screw apart from many other, arguably more familiar screw head drive styles - products such as the classic cross/Phillips head screw, the universal slotted/flathead version, and the increasingly common Pozidriv (star-shaped) variants.

Another common distinction is that socket screws are often squared off at the tip; these are generally referred to as flat point socket screws, and they tend to be the most widely used type found in many day-to-day applications. Note, however, that various other designs - such as cone point, cup point and dog point socket screws - are also available, as we’ll see further on in this guide.

In most other regards, including such cosmetic details as general size, shape and finish options, there isn’t typically much physical difference between socket screws and regular screws for the majority of everyday uses. So what exactly is the point of a socket screw, and when should you use one?

In short, the design of a socket screw is effectively an attempt to combine some of the increased performance features of other common hex fasteners - nut and bolt systems being the obvious point of reference - with the convenience and ease of use of a standard format screw set. This middle ground can be a useful compromise in many situations, particularly when working in spaces that don’t allow sufficient space or ease of access for securing externally wrenched hex fasteners (again, bolts would be the obvious comparison here).

Advantages of socket screws

The potentially higher torque and clamping force that can be achieved with a socket screw over a regular screw makes them a preferable option in some common use cases. When rated up against regular screws and bolts in a like for like test, socket screws can often deliver some or all of the following advantages in relevant scenarios:

• Since socket screws typically offer greater clamping force than standard screws and many other small-format fastener types. This means that fewer socket screws can be used in a given application to achieve the same overall clamping force on the joint or workpiece.

• Using fewer screws means:
  • Reduced hardware costs
  • Fewer holes to drill and tap into mounting or mating surfaces, saving time and reducing potential damage to components
  • Lower overall weight of the finished piece

• The trend towards lightweight constructions when using socket screws is further enhanced due to their smaller individual size than many other screw types and other fastening components - especially compared to nut and bolt fixing arrangements. In particular, the cylindrical heads and internal drives of socket screws are far smaller in diameter than fasteners with external hex heads, and therefore need less space (with no additional room needed to manoeuvre a wrench) for installation

What is a socket head cap screw?

The generic name 'cap screw' was traditionally used as something of an umbrella term, meaning any of various screw types chosen for fastening applications where the use of a nut and bolt installation was deemed unsuitable. However, today it tends to refer specifically to a particular style of screw head.

Any number of different screw types and shapes can incorporate elements of the basic cap screw head design - you might find anything from shoulder screws and countersink screws, through to button and fillister head screws, described broadly as 'cap screws'. A socket head cap screw is also commonly referred to as a socket cap screw, Allen bolt, or simply a socket screw (in many ways, it's very much the 'standard' version of a socket screw).

Socket head cap screws are very widely used along all manner of industrial production and assembly lines, including in the automotive sector, cabinetry and furniture manufacture, machine tooling applications and steel fabrication. This is chiefly thanks to the fact that they’re easy to install and tighten/loosen, as well as providing impressive clamping strengths while retaining a discreet and tidy finished look.

As with the vast majority of all socket screws, the socket head cap screw features a cylindrical head with an internal hex drive hole, and is most often (although not always) threaded all the way along the full length of the shank. And, also like nearly all socket screws, they're primarily designed to be used in assemblies where there isn't sufficient clearance to install a conventional nut and bolt, or to manoeuvre a standard socket wrench.

Socket screw sizes

The average socket cap screw head is around 1.5 times the diameter of the screw shank, and about as high as the shank is wide. However, alternative head designs for socket head cap screws include low head, button head and flat head varieties, intended for driving into countersunk holes and other application-specific mounting positions.

When looking to buy socket screws, note that the product’s stated length is typically measured from immediately under the head to the tip. (This is the case for most screw types, but some specific varieties - thumb screws, for example - will often give stated length measurements that include the height of the head in the overall specification.)

Most socket screw types are available in either metric or imperial measurements (or both) for length and diameter, depending on the particular manufacturer or supplier in question. Many leading brands of socket screw manufacturers, will often provide suppliers with a socket screw sizing chart or similar tool that can be referred to at the point of purchase.

While relatively small socket screw sizes (of diameters ranging from M2 to M6, and lengths of around 3mm to 20mm) tend to be more in demand across a wider range of tasks and applications, the full range of available sizes is considerable. Larger, heavier gauge and extra long socket screws are not at all uncommon, particularly in widths of around M8 to M20, and lengths of anywhere up to 130mm.

Extra long socket screws are especially frequent requirements for use in electrical plug sockets. Common electrical socket screw sizes include 40mm, 50mm, 60mm, 75mm and 100mm.

Socket screw materials

There are many different materials used in the manufacture of socket screws, including the very common steel and stainless steel varieties, as well as brass, aluminium, black nickel or zinc-plated and even chrome socket screws. The ideal material to choose will depend entirely on the specific application you’re intending to use the socket cap screw set in.

Aluminium Socket Screws

Aluminium socket cap screws are often preferred for applications in which weight is a key consideration - these might include anything from the automotive industry through to domestic uses such as installing hardware and trim on moveable furniture, cabinets, doors and windows.

Despite only being around one third the weight, aluminium is durable, but rather less so than steel, and tends to have lower shear strength in particular. While aluminium offers decent performance in terms of corrosion resistance in more challenging environments, it’s less hardwearing in this regard than stainless steel.

Aluminium socket screws are also typically seen as unsuitable for use in most kitchen utensils and food preparation/production line roles. Stainless steel is again preferred here, due to aluminium’s ability to influence the flavours and appearances of certain foodstuffs under the right (wrong?) conditions.

Other socket screw materials are detailed below.

Types of socket screws

As well as different materials, socket screws are also available in different types:

Top picks

There’s a wide range of types to choose from – browse our top brands by clicking the link to view the full product range.