Thread Standards – For Professionals by Professionals

The most commonly used thread standards are the metric ISO thread, abbreviated as M, and the American UN thread, abbreviated as UNC, UNF, etc. In addition, there are other thread standards that have developed and continue to be used in different application areas. These include specific standards for threads in various industries, as well as in aerospace. An example of this is the MJ thread.

The most frequently used types and standards for threads are as follows:

  • The metric ISO thread according to DIN 13
  • The Unified National (UN) threads, including UNF, UNEF, UNC, among others
  • Trapezoidal threads, such as ACME and TR
  • Various Whitworth threads, including BSF and BSW
  • Pipe threads, such as the American pipe thread NPT and the Whitworth pipe thread G (BSP)
  • Buttress threads
  • Round threads

The thread types and standards are further divided into movement and fastening threads. Fastening threads are used for securing and screwing components together. A clear example of this is screws and nuts that come with a fastening thread, such as the metric ISO thread M.

On the other hand, movement threads are designed to transmit movement or to move something. A good example of this is the trapezoidal thread, which is often used in spindles. The designation of the threads typically begins with the identifier for the thread standard, followed by the nominal diameter, which often refers to the external diameter, as well as the pitch and the number of individual threads per inch. At the end, the tolerance, sometimes the taper, the direction of rotation (LH for left-hand thread and RH for right-hand thread), or the multiple-start feature are specified.

The drawing indicates that the largest diameter is represented by the external diameter. In contrast, the diameter that extends from the bottom of one thread to the opposite thread bottom is the core diameter. The flank diameter occupies the middle between the core and the external diameter. The pitch refers to the distance from one thread peak to the nearest thread peak. For threads per inch, referred to in English as TPI (Threads Per Inch), the thread peaks that are within one inch (25.4 mm) on a thread length are counted.

On our online shop at, you can find all thread gauges, taps, thread cutting dies, and thread milling cutters for all typical thread standards.