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Socket screws explained

clock 7.5 minutes | 27 Jan 2022

Socket screws in use in an engine

Socket screws contain a hexagonal socket, or internal drive, in the head. The way to tell if a screw is a socket screw is by the tool required to tighten and loosen it. The general rule of thumb is simple: if you need an allen or hex key, or a special internal wrenching drive, then it can be classed as a socket screw.

Socket screws are known by different names:

  • Socket head cap screw, or socket cap screw
  • Hex socket head cap screw, or hex socket screw
  • Allen socket bolt, or allen bolt
  • Socket head screws

Hex socket head screws vs hex head screws

What is a hex head cap screw? Is it the same as a hex socket head screw? The names are similar and shouldn’t be confused. You can see the differences here:

Hex socket head screw

  • Only the socket is hex shaped
  • Requires an allen, or hex, key for adjustments
  • Ideal when space is limited

Hex head screw

  • Solid hex-shaped head – no socket
  • Requires wrench for adjustments
  • Room needed to use wrench

Hex socket drives resist cam-out better than other drives. You can learn more about the different screw types available in What are screw heads, drives and threads?

The advantage of socket screws

Socket screws are the preferred screw when bolts and nuts can’t be used. The hex socket means the screw has six stress-bearing sides. Compared to other screws, socket screws of the same size enable you to achieve higher torque and clamping force. This, in turn, allows you to use fewer socket screws than you’d have to install if using other screws.

Fewer pieces of hardware reduce the weight of your finished application. If that’s a priority – think automotive – then socket screws are ideal. Fewer screws needed also reduces your costs and time needed to drill holes and tap into surfaces.

Types of socket screws

There are several types of socket screws to choose from. While standard socket screws are easy to find, you could also consider non-standard socket screws. These are screws modified in some way from established standards. Our team of experienced engineers can design and prototype screws from you from concept. Learn more about our custom solutions.

Hex socket head cap screws – standard

The head is 1.5 times the nominal thread diameter, making it a smaller diameter than most other machine screw heads. When in use, this screw is recessed within a counterbored hole so that the head is flush with the surface of the component being fastened. This result is a clean appearance. There are no protrusions on the socket cap that could catch skin or clothes, which is why they’re often used in sporting equipment.

Hex socket head cap screw - standard

2. Button head socket cap screws

Button head socket cap screws

Hexagon socket button head cap screw are sometimes referred to as dome or round screws. They’re characterised by their wide, load-bearing surface, which helps them resist loosening. The look is popular for aesthetic purposes, while the lack of sharp edges gives these screws a measure of safety. The bulbous head allows more torque than normal to be applied for greater clamping force. For best results, use with a retaining washer to lock in place.

Button head socket cap screw

3. Countersunk socket screws

Also called hexagon socket countersunk head screws. The shoulder slopes at an 82-degree angle if imperial screws and around 90 degrees if you’re working with metric screws. The flat head socket is designed to sit either flush with its surface or just below. Applications vary, depending on the countersunk socket screw dimensions. For instance, the shorter variety are ideal for light-duty applications that need a strong joint. Otherwise, you’ll find these screws used in the automotive industry, industrial equipment, fixings and electronic applications.

Countersunk socket screws
Hex-socket set screws

Used to generate a compressive load, which is opposite to a fastener tensile load created by bolts and most other screws. Socket-hex set screws hold parts beneath the cup – the concave, hollowed end – resisting rotational or lateral movement and standing up to vibration. When choosing your socket screw size, keep in mind that the screw diameter should be roughly one half of the shaft. Set screws are intended for light loads. For extreme conditions, socket head cap screws are your best bet.

Hex-socket set screw
Hex-socket shoulder screws

The partially smooth shaft enables some rotation and movement around the screw axis while the threaded end will usually use a nut to secure the screw's position. Socket shoulder screws are typically used when pre-tapped holes need alignment with some tolerance. Used in equipment and machines with components that require the ability and room to swing, slide or pivot.

Hex-socket shoulder screw

Which materials?

Socket screws are available in steel, stainless steel, aluminium, brass and plastics. For strength, plastic socket screws are usually made of nylon, nylon 6/6 or PEEK® and are excellent replacement screws for metal.

Aluminium socket screws are lightweight, which makes them an ideal choice for automotive applications – especially when you consider that it has the best strength to weight ratio of all common metals. Aluminium also resists corrosion. However, in tough environments, stainless-steel socket screws will do a better job when it comes to standing up to oxidation.

Steel socket head cap screws are often plated with zinc or nickel for corrosion resistance.

Steel is an expensive option due to its resilience and durability, so first make sure another material can’t do the job more cost effectively.

Solid brass is more corrosion resistant than stainless steel, so this is a great option if your application is outdoors, such as doors or garden furniture. A brass plated material is not as strong against corrosion, which is important to in mind. Brass also has aesthetic appeal, but it will require upkeep. While brass resists corrosion, no material can withstand oxidation 100%. Black tarnish can appear if not properly maintained.

Materials at a glance

Stainless steel
Zinc- or nickel- plated steel

Corrosion resistant

Sizing your socket screws

For the vast majority of screws, including hex socket caps, length is typically measured from just under the head to the tip. This is the case whether we’re talking about short or long socket screws, and even extra-long socket screws, which are usually used as wall socket screws.

Like most screws, sockets are available in either inches, called standard, or metric measurements for length and diameter. You can learn more about sizing, along with standards, by checking out What is the difference between metric and standard fasteners? We explain how to measure screws in What are screw heads, drives and threads?

Download free CADs and try before you buy

Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the threaded socket screws you’ve chosen are exactly what you need. You might find The ultimate guide to fasteners helpful. If you’re not quite sure which product will work best for your application, our experts are always happy to advise you.

Whatever your requirements, you can depend on fast despatch. Request your free samples or download free CADs now.


Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.

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