Having the right threads in a bolt is critical to its function and success. Threads are generated by a subtractive process, which can be applied to a wide variety of hardware materials and processes (including various cutting [single-pointing, taps and dies, milling]; molding; casting [sand casting, die casting, and sand molding]); forming and rolling; grinding; and annealing). This means there are many different options for the thread shape and size, which also has a big impact on performance. The right thread size and pitch helps ensure that the nut and bolt can fit together without being stripped or seizing.
The most common measure used for threaded hardware is called threads per inch, abbreviated as TPI. However, TPI can be a bit confusing because it refers to both the number of threads on the bolt and the distance between them in inches. In order to accurately determine these measurements, you must know the thread diameter and the thread pitch.
You can use a caliper to measure the distance from one thread peak to the next, but this is a difficult and time-consuming method. A much simpler and more accurate way is to use a thread gauge, available at most home improvement and hardware stores. These have a serrated edge stamped with measurements, and by looking at the number that sits tightest on the gauge you can easily get both TPI and the thread pitch.
Both the thread diameter and the pitch must be determined before you can correctly identify a bolt or screw’s size. The easiest way to do this is with a caliper, but it’s also possible to use a ruler and count the distance between thread tips. For metric bolts, you can find the thread size by dividing the thread pitch by the number of threads.
Thread pitch is the distance between adjacent thread peaks, and it’s usually expressed in mm. For example, a bolt with a standard coarse thread has a pitch of 0.7 mm. The thread diameter is generally expressed in inches, while the thread pitch is expressed as a number followed by a hyphen and then the metric measurement in millimeters.
The major diameter of a bolt is the distance from the end of the unthreaded portion to the beginning of the first threaded portion. This is the dimension that’s commonly used for identifying the proper bolt length, and it’s also sometimes referred to as the “major thread.”
The minor diameter of a bolt is the distance from one end of the unthreaded portion to its closest point on the surface of the threaded portion. This is the dimension for identifying the correct thread width and depth. Threads with a finer pitch are stronger in both tension and shear than threads with a coarser pitch, and they require less torque to develop an equivalent bolt preload. This feature makes them better suited for applications with hard materials and thin-walled tubing. In contrast, coarser threads require more force to achieve the same result and are often less able to handle vibration. bolt threads per inch