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What is a Cap Screw?

Dorothy Distefano
Dorothy Distefano

A cap screw is a type of fastener used for making mechanical connections between mating objects, to ensure that they are held together securely. These screws are commonly used to fasten machine parts and many other types of objects, such as those inside home appliances or consumer electronic devices. It is important to select the right size and type of cap screw for each individual application.

A cap screw is directly tightened into a threaded or tapped hole, and is normally used without a nut. It has a large head on one end and a cylindrical shaft with an external thread — a helical structure that allows the screw to be advanced when rotated. The tapped hole has an internal thread that matches the external thread of the screw. When this screw is inserted and rotated into a tapped hole, it advances. The screw is tightened and loosened by applying torque to the head of the screw using a tool.

Allen wrenches, which are used with hex head cap screws.
Allen wrenches, which are used with hex head cap screws.

A variety of cap screw heads are available to allow different tools to be used for tightening or loosening the screw. The head has a larger diameter than the threaded portion to provide a positive mechanical stop when tightening the screw, and to allow the head to be shaped to accept a specific type of tool. Examples of commonly used cap screw heads are the hex head, which has a hexagonal shape for use with a spanner wrench; the socket head, which has recessed hexagonal hole for use with an Allen wrench; and the button head, which has a lower profile, dome-shaped head and recessed hexagonal hole for use in counterbored holes.

Socket-head cap screw.
Socket-head cap screw.

A cap screw can generate a high amount of clamping force when tightened. The mating surfaces between the screw and nut resist the force being applied by the screw, and help to prevent the mechanical connection from loosening over time. The type and size of cap screw selected for a particular application depends primarily on the forces required to adequately secure the mechanical connection.

Cap screws are available in both English and metric sizes, and with a wide variety of coarse and fine threads. They can be manufactured using carbon steel, stainless steel, or brass for corrosion resistance; metal alloys for high-strength applications; and even plastic materials. When selecting a cap screw, it is important to understand the environment and stresses that the mechanical connections will be exposed to, so that the proper size and material are used.

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Discussion Comments


A cap screw, by definition, is a machine bolt or screw that is threaded the to the cap and is normally fastened to a threaded hole rather than a nut. The picture of the "socket-head cap screw" accompanying this article would actually be a socket-head bolt, according to traditional definition.


I needed to buy screws the other day, and I didn't realize how much variety there was. I was kind of overwhelmed.

I wasn't aware of the number system, or that might have been able to help me. Besides knowing the diameter you need (which was what I did know), you have a lot of options of the number of threads per inch, the size and shape of the head, and the screw material. Luckily I just needed some basic screws, so I wasn't too worried about making a perfect choice, but I'll definitely plan ahead next time.


@JimmyT - The numbers on screws represent the gauge of the screw. The gauge refers to the diameter of the metal rod that was used to make the screw.

Oddly enough, the system used for screw gauges is the complete opposite compared to wire gauges. The larger a screw gauge gets, the larger the diameter of the screw becomes. With wire, smaller numbers are thicker, and larger numbers are thinner. I'm not sure where the conventions for these came about.

You can find reference charts online with the specifications for several screw types, but Number 5 screws are a quarter inch, and Number 14 screws are a half inch.


@jmc88 - I personally can't think of any other types of screws that wouldn't use this system.

The only thing I can think of is whether bolts are considered a type of screw. Since they are turned using a socket and need a nut to stay in place, they might fit, but it seems like bolts wouldn't really be considered a type of screw.

Out of curiosity, whenever I buy screws, they are always numbered depending on the size. I'm not talking about a length in inches, but a screw number. For example, the other day I needed screws so I went to the hardware store and bought a box of Number 5 screws. Where does that number come from?


So from what I understand, a cap screw is just another name for a regular, everyday screw. Is that right? I've never heard it called that, but I guess the term cap could refer to the fact that this is a head on the screw that can be turned with a screwdriver.

I suppose that begs the question of what are the names of some of the other types of screws? The article mentions that cap screws do not usually have a nut on them. Is there a name for screws that usually do need a nut?

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    • Allen wrenches, which are used with hex head cap screws.
      Allen wrenches, which are used with hex head cap screws.
    • Socket-head cap screw.
      Socket-head cap screw.