Machine Elements & Parts

What is the Thread Major Diameter?

Screw threads have a very important place in our life. There are different kinds of screw thread geometries that we are using. But if we take a look as an engineer at the screw geometries, there are various kinds of geometric terminology that we need to know about the threads. One of these terminologies is the thread major diameter. Here you can find important information about the screw terminology that you need to know in general.

What is Thread Major Diameter?

Thread major diameter.
Image Source: Wikimedia Commons.

Thread major diameter is a geometric term about thread geometry. It states that the maximum diameter of the external thread geometry that we are measuring is from the tip of the crest. In terms of the mechanical assemblies, it is a very important parameter that we need to consider.

Why Thread Major Diameter is Important?

The importance of this term comes from mechanical assemblies. We need to consider the major diameter value while we are designing the holes and threaded holes that we attach to the fasteners. If we design a hole smaller than the major diameter, the screw will not fit inside the hole. So, you need to adjust hole tolerances according to the major diameters that you have.

Because of this, if you look at the screw thread catalogs, you can see the important information about the thread major diameters. You need to consider them correctly while you are making your designs.

Other Important Screw Thread Terms

Like thread major diameter, there are also other important terms that you need to know about screw threads.

  • Minor Diameter: This is the smallest diameter of the screw thread. We measure this value from the root of the screw tooth.
  •  Pitch Diameter: This is an also imaginary diameter that we are using for the screw threads. It is an imaginary cylinder, the surface of which would pass through the threads at such points as to make equal the width, the threads, and the width of the spaces cut by the surface of the cylinder. Pitch diameter is also a very important parameter when we select the different sizes of external thread geometries.
  •  Pitch: This is the distance between the point on a thread to the corresponding point to the next thread. It is an important factor that defines the number of teeth in inches or millimeters.
  •  The angle of Thread: The angle of the thread according to the axial plane of the screw.
  •  Crest: The highest point of the screw geometry which we take geometric measurements from that point for the thread major diameter.
  •  Root: This is the geometric lowest point on the screw geometry that we take as a reference while we are measuring the minor diameter.

These are also very important terms that you need to consider while you are looking for thread geometries from catalogs.

Conclusion on Thread Major Diameter

As you see above, thread major diameter is a very important factor. There are different standards that industries are using for these values. The most common standards are the metric and ANSI standards.

If you have further questions about this topic, leave them below!


Is the major diameter the same as the nominal diameter? 

Yes, they are representing the major diameter of the external threads. You can see the different kinds of catalogs as nominal diameters of this value.

Why nominal diameter is used? 

The general cause is, it gives the maximum diameter of the external thread and screws. The designers can easily asses if they can use this geometry in the whole design that they have.

Uğur Uygun

Hello everyone, I am Uğur. I am a mechanical engineer who has experience in different R&D departments of HVAC companies. With my best friend, I decided to build this website to share out knowledge and experience. And also, I am interested in automotive. So, I am writing about automotive and automotive-related articles here. Sharing my knowledge and experiences on these topics is a very big pleasure for me. I am willing to answer all of the questions people have about these topics.

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