Understand The Threaded Hole Callout Standard

Units on Hole Callouts — Onshape

Threaded holes are used in many engineering and manufacturing applications. They are used to secure two or more parts together with a fastener, such as a screw or nut-and-bolt combination. A threaded hole callout is a way to specify the size, thread type and other important characteristics of a threaded hole. Threaded hole callouts are a series of letters, numbers and symbols that are used to accurately identify a threaded hole. Understanding threaded hole callouts is essential for any engineer or machinist who will be working with them.

The Basics of Threaded Hole Callouts

Threaded hole callouts are often misunderstood, but they are relatively simple. The most basic callout includes three elements. The first element is the diameter of the hole in millimeters or inches. This is followed by the thread pitch in millimeters or threads per inch. Finally, the callout includes the thread type, such as an ISO metric thread, an American National Standard (ANSI) thread or a Unified National Thread (UN). In some cases, the callout may also include the length of the threaded hole.

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Additional Components of Threaded Hole Callouts

In some cases, a threaded hole callout may include additional elements. These include the thread angle, the class of fit, the thread length and the thread direction. The thread angle is the angle of the thread, expressed in degrees. The class of fit is a measure of the fit tolerance of the thread; it is expressed as a number or letter. The thread length is the total length of the thread, measured from the start of the thread to the end. The thread direction is the direction in which the thread is rotated as it is inserted into the hole.

Standard Threaded Hole Callout Notations

The threaded hole callout standard is an internationally accepted standard used by engineers and machinists. It is important to understand the standard in order to accurately identify a threaded hole. The standard includes the diameter, thread type, thread pitch and thread angle. It also includes the class of fit, thread length and thread direction, if applicable. In some cases, the standard also includes the minimum thread engagement and the minimum thread diameter.

Using Threaded Hole Callouts

Using threaded hole callouts is relatively easy. Simply measure the diameter of the hole and the thread pitch, and then match those to an appropriate threaded hole callout. If additional elements are needed, such as the thread angle, thread length or thread direction, these can be determined from the standard. Once the threaded hole callout is determined, it can be used to accurately identify the threaded hole.

Tolerances and Threaded Hole Callouts

Tolerances are important when using threaded hole callouts. Tolerances are the allowable deviation from the standard. In most cases, the tolerances are expressed as a plus or minus value in millimeters or inches. The tolerances should be taken into account when using threaded hole callouts, as they can affect the accuracy of the threaded hole.

Common Threaded Hole Callouts

There are many different types of threaded hole callouts, and it is important to understand the most common ones. The most common threaded hole callouts include ISO metric threads, American National Standard (ANSI) threads, Unified National Threads (UN), British Standard Threads (BSP), and Whitworth threads. Each of these threaded hole callouts has its own set of specifications that must be followed.


Threaded hole callouts are an important tool for engineers and machinists. Understanding the threaded hole callout standard is essential for accurately identifying threaded holes. It is important to understand the basic elements of threaded hole callouts, as well as additional elements such as the thread angle, class of fit, thread length and thread direction. In addition, it is important to understand the standard threaded hole callout notations and the tolerances that should be taken into account when using them. Finally, it is important to understand the most common threaded hole callouts.