Cap Screws in Machines; Types and Specifications

In machinery, designers are using different kinds of screws and threads. One of these screws is the cap screws. Furthermore, they are very common in mechanical design. Here, you can find different information about this screw type.

Cap screw.
Image Source: Wikimedia.

What is the Cap Screw?

They are the screw type in which the threads start from a distance after the surface of the cap. Also, the cap means, there is a head-on these screws that we attach screwdrivers to montage or de-montage it.

In general, we are using cap screws in the assembly of multiple layers of parts. The first layer generally lies in the threadless section beneath the cap. And the second layer, where the threads penetrate. This is the most general use of them.

Types of Cap Screws

According to the shapes of the heads, there are different types of cap screws. For all standards, the size proportions of each type are the same.

Hexagon Head

Hexagon head cap screw.

This is the most common type of them. The height of the head is equal to the 2/3 times diameter of the screw. The total clearance of the hexagon circle is equal to 1.5 times the diameter.

For attaching and detaching the hexagon head screws, you need to use wrench sets.

Flat Head Type

Flat head cap screw.

The most important advantage of the flat head type is you can attach it to the work part witohout giving a protrusion of the cap. The top of the cap is at the same level as the surface.

The total cap angle of the flat thread cap screw is 80°. The screwdriver clearance on the cap is 0.25 of the diameter. The deepness of the screwdriver clearance is also 0.25 of the diameter.

You can attach or detach this type of them with regular screwdrivers.

Round Head Cap Screws

The difference between the round head cap screws and flat head ones, the head of the screw is half sphere shape. There is a screw clearance also on the head of the screw.

The height of the head is equal o 3/4 times body diameter. The screw clearance is 0.25 of the diameter and the deepness of that is 0.5 of the diameter.

Also for round head them, you need to use regular screwdrivers.

Fillister Head Type

Fillister head also has round head shapes. But this round hear is buried inside the work part. There is very little protrusion on the surface.

The height of the head is equal to 2/3 times body diameter. The diameter of the head is equal to 1.5 times the diameter of the body. The screwdriver clearance is the same as the flat head type.

Also, you need to use the same screwdrivers to detach them.

Hex Socket Type

Hex socket type.

These types of screws are different from the hexagon head. The head of the screw is buried inside the work part. There is a little protrusion on the surface.

You need to attach a hexagonal socket to detach or attach the hex socket type.

The total height of the head is equal to the body diameter. And also, the socket clearance is equal to half of the head diameter.

Advantages and Disadvantages of Cap Screws

Like the other assembly types, there are various kinds of advantages and disadvantages of cap screws.


  • The most important advantage of them is the simplicity of the construction. They are very simple to attach and detach various parts mechanically.
  • In general, we can obtain disassembling devices and systems with cap screws. There is no need to damage the main parts to disassemble these screws.
  • You can easily change them with new ones in the maintenance operations.

General Disadvantages of Cap Screws

  • The most important disadvantage of them is their mechanical stability and strength. If we compare the cap screws with the welding operations, there is a great difference in the mechanical strength. In the welds, the stress and force spread to all weldments. But in these mechanisms, all the stress is at the screw assembly.
  • Self-disassembly can be a problem for them. Because of the vibrations and other mechanical factors, disassembling in time may be a problem.
  • To use them in mechanical assemblies, we need to create proper holes which can be an additional cost. Also, we are damaging the integrity of the work part.

Specifications of Cap Screws According to the Standards

According to the different standards, we can specify the different cap screws in technical drawings. You can learn how to note these screws on technical documentation and engineering drawings with the examples below.

Specification of US Screws

  • .350 – 15 UNF-2B X 2.5 HEXAGON HEAD CAP SCREW

In the example above, it is very simple to take information about screw type.

.350 defines the body diameter of the screw in inches. 15 UNF-2B is the specification of the thread on the cap screw. You can learn it by clicking on the given link. HEXAGON HEAD CAP SCREW gives the type. Also, this indication can be like this; HEX HD CAP SCR. It is more useful in technical drawings.

Specification of the Metric Screws

Metric screws are also very common. You can learn how to note the metric ones by checking the example below.

  • M15 X 3 X 80 HEX HD CAP SCR

So, it is very simple. M15 defines the metric of the screw. It is the nominal body diameter of the cap screw. 3 is about the thread length and the 80 is the length of them in millimeters. Also, you know that HEX HD CAP SCR gives information about their type of them.


So, cap screws can find very important applications in machine design. Also, there are different types of these screws. These different types can be used in different kinds of applications.

You need to be aware of the advantages and the disadvantages of them in general. Because there are also other types of assemblies.

Specifications are also very important. So, you need to understand which kind of them are used in typical machinery.

Above all, do not forget to leave your comments and questions below about this topic.

Your precious feedbacks are very important to us.

FAQs About Cap Screws

What is the cap screw? 

They are the mechanical element that has a thread to penetrate the workpieces. So, they are used for the assembly of multiple layers. There is clearance at the cap of the screw where you can assemble and disassemble the screw.

What are cap screws used for?

Also, in nearly all mechanical applications, they are using them for the assembly of the machinery.

Are all socket cap screws grade 8?

No, there are different grades of socket cap screws on the market. And also, socket-type screws are generally used in heavy jobs. Because of it, grade 8 is the most common grade of socket screws.

How the cap screws are manufactured?

Manufacturing them is very simple. So, to create the raw head and body of the screw, generally forging and hot metal shaping operations are used. After that, with the turning or other machining operations, they obtain the thread geometries.

What are the types of cap screws?

There are different types of them; hexagon head, flat head, round head, fillister head, and hex socket types are the main types.


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