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Threaded Rod Dimensions: Everything You Need to Know

Posted by Keystone Threaded Products on Jun 9, 2021 11:11:27 AM



In our last blog, we provided an overview of the types of threaded products available and their specifications. In this blog, we will provide more detail about threaded rod dimensions, so you can make more informed decisions when selecting threaded rods.

Major, Minor, and Pitch Diameters

As with any component that has a complex geometry, threaded rods must be regularly inspected for compliance. 

Each type of threaded rod has a standard that is specific to it. These standards contain the associated major, minor, and pitch diameter requirements.

    • Major diameter: The largest diameter of the screw thread, measured from thread crest to thread crest.
    • Minor diameter: The smallest diameter of the screw thread, measured from thread root to thread root.
    • Pitch diameter: The simple, effective diameter of the screw thread, approximately halfway between the major and minor diameters.


All diameters are measured in either inches or millimeters. When specifying a threaded rod, the major diameter should be used. This can be expressed as a fraction, decimal, or machine screw number. 

In addition to the diametrical measurements, the standards mentioned above include linear characteristics such as: the thread's profile, pitch, and lead variance.

Thread Pitch

Thread pitch is often referred to in terms of threads per inch (TPI). TPI means that for a given thread pitch, there are X number of threads included within the linear span of an inch. 

Oftentimes, thread pitch is another way of expressing the number of threads per inch (e.g., “4-Pitch,” “8-Pitch,” etc). Thread pitch is the phrase used in technical terms as the distance of the center of one full thread profile to the center of the next. 

Thread pitch may be calculated as 1/TPI. It is measured by the use of a thread ring gauge, thread plug gauge, thread pitch gauge, optical comparator, or any combination of these tools.

Classes of Fit

Unified threads, which are the most widely used thread form in the U.S., are broken down into classes 1, 2, and 3. Each step up in the numbering system requires a closer fit between the external and internal thread during engagement. 

It is important to select the same class for each internal and external thread. A thread's class limits the clearance between mating components with each step up. A class 3 thread is that with the closest fit. The closer the fit, the less clearance there is to allow movement while the threaded components are engaged. 

Additionally, as the fit becomes closer, it is ever more crucial that the thread profile be absent of other defects, such as burrs, compression marks, and other means of interference. This is because there is less forgiveness inherent in the clearance of the mating threads.

Standard Threaded Rod Lengths

Threaded rods typically come in 3', 6', and 12' lengths. Overall length is subject to customer requirements and may be cut to length.

For threaded rod length, it is important to identify whether the overall length (the entire length of the product, from end to end) of the product is being measured, or whether it is measured first thread to first thread.

When a threaded rod or other threaded component consists of threads on either end and an area without threads somewhere between them (as is the case with stud bolts), it is measured first thread to first thread. This is the trans-axial distance along the major diameter of the threaded component from the start of each opposing thread.

When specifying a threaded rod, the length comes after the class of fit. The two are typically separated by an “x” (read as “by”).

Find the Right Threaded Rods for Your Specific Needs

Now that you have a basic understanding of key threaded rod dimensions, you can be confident knowing that your order will fulfill your application’s requirements. 

If you have questions about any of the information in this post or would like to start a conversation about your next project, fill out the form below. We would love to hear from you.

Topics: Acme Thread, Unified Thread, Trapezoidal Thread, Threaded Rod Dimensions

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