What Are Bolt Threads Per Inch?

Bolt threads per inch refer to the number of threads in a single inch of the bolt’s length. It is also known as the “thread diameter.” A bolt’s thread diameter is determined by measuring the distance between the peak of one thread and the top of the next one. This measurement is taken using a caliper. The higher the number, the finer the thread. Fine threads allow for more precision and require less torque to achieve an equivalent bolt preload than coarse ones.

The threads on a bolt are uniform bands of protruding material that helically rotate around the screw’s shank. When viewed horizontally, the threads can be clearly seen and counted by eye. A numbering system was developed to make it easy for people to count and identify fasteners by their characteristics, such as threads per inch. In the United States, the majority of threaded hardware is still manufactured to UTS (United States Standard) sizes and standards. This hardware is still commonly called by its USS or SAE designations, even though these are superseded by the metric sizes of ISO (International Organization for Standardization).

Metric threads have different dimensions, but the general principles behind their measurement and identification are similar to those of the American Standard threads. The most significant difference is that metric threads have pitch measurements expressed in millimeters, while the American Standard threads use inches. In a metric thread callout or the label on a metric threaded fastener, the pitch is stated as the number after the x symbol. For example, a metric M10 x 1.25 bolt has a thread pitch of 12.25 millimeters.

In addition to the thread pitch, there are three characteristic diameters (Dmaj, Dmin, and P) that are used to describe a threaded fastener:

While coarse and fine threads share the same basic profile, the terms don’t necessarily imply anything about the tolerances used, craftsmanship, or quality of the fastener. In fact, there are high-quality, low-tolerance hardware manufacturers that produce fine threads that are suitable for some of the most demanding applications.

The major diameter, or crest diameter, is the largest and outermost diameter of the screw’s shank. The minor diameter is the thread’s smaller, innermost dimension. The pitch, or distance between the major and minor diameters, is defined by the width of a single thread when it extends from the base of the root to the tip of the crest.

To measure thread pitch, you can simply use a caliper to check the distance between two thread peaks. However, it is more efficient to use a thread pitch gauge that can accurately determine the number of threads in a given length. This is the most accurate way to determine a bolt’s threads per inch, especially for metric fasteners. For instance, the quickest and most accurate way to determine the TPI of a 10mm M10 metric bolt would be to use a thread pitch gauge and count its threads. Since the metric system is eclipsing the American Standard System, most of the world now uses the metric threading standard. However, it is not uncommon for manufacturers to mix the two systems in order to meet customer demand. bolt threads per inch

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