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What is a Major Diameter?

By B. Turner
Updated May 17, 2024
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Major diameter is a measurement used to identify the size of a screw or bolt. The major diameter of these fasteners represents the distance around the outer portion of the threads, which is the widest portion of the shank. This measurement should be taken around the male threads, and can not be taken using the female portion of the threads or the head of the fastener. Major diameter plays a key role in most common screw identification systems, including both metric and non-metric systems.

Using different measurements on any given screw, users can identify the type of screw and how it can be used. Suppliers and manufacturers provide charts that enable users to find different screw measurements using known figures or features. These measurements may include the major diameter, or minor diameter, which represents the size of the female threads along the shank. These charts may also include pitch, which represents the number of threads per inch or centimeter measured along the length of the shank. By combining two or more of these measurements, users can identify a loose screw.

The major diameter of a screw or bolt can be measured using a hand tool known as a micrometer. Depending on region, users may require a metric or non-metric micrometer tool. To obtain an accurate measurement with this tool, it is critical to only measure around the widest portion of the threads on a screw. In some cases, excess wear may have reduced the size of the threads, resulting in imprecise measurements. Burrs or voids along the edges of the threads can also impact the accuracy of the reading.

Using major diameter and other measurements, workers can identify the type of screw used on a particular project. This is helpful for matching the existing screws to create a secure and cohesive connection. With so many shapes and sizes of screws available, knowing the major diameter and one other measurement makes it much easier to choose the right screw at the store or in the workshop.

While people throughout the world use many different systems to identify types of screws, two major systems serve as the most widely used methods of screw identification. These include the Unified Thread Standard (UPS) and the International Organization for Standards (ISO). Under UPS identification standards, screws are identified by a number that can be used to identify the size and pitch of the screw. To find the major diameter of a UPS screw, multiply the identification number by 0.13, then add 0.60, which gives you the diameter in inches. Under ISO identification standards, the major diameter is given as the first number in a sequence of numbers used to identify each screw.

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