20-UN Thread (USA)

The 20-UN thread is an American thread standard with an unchangeable pitch. The 20-UN thread is just one of several such threads in the UN series, which also includes threads with constant pitches of 4, 6, 8, 12, 16, 28, or 32 threads per inch. For all these threads, varying outer diameters are combined with the always the same pitch, as stated in the name. The dimensions of the 20-UN thread are set in the ANSI/ASME B1.1 standard, designed by the American Society of Mechanical Engineers (ASME), and monitored by the American National Standards Institute (ANSI).

The flank angle of the 20-UN thread is 60°, and all dimensions are given in inches. The thread designation consists of two components. First, the outer diameter must always be stated, followed by the abbreviation 20-UN. The constant pitch is already part of this abbreviation and therefore does not need to be stated separately. This results in a thread identification of, for example, 5/8″ – 20-UN.

For all UN threads with constant pitch, it is important to note that they only include combinations of outer diameter and pitch that are not already part of the common standards UNC, UNF, or UNEF. For example, a thread with 20 threads per inch and a 1/4″ outer diameter already exists in the UNC thread (1/4″ – 20 UNC). Consequently, the outer diameter of 1/4″ is not listed in the 20-UN thread table.

The thread table for the 20-UN thread also includes only outer diameters that are sensibly combined with a pitch of 20 threads per inch. Specifically, this means a selection ranging from 5/16″ to 3″. In the table below for the 20-UN thread, you will find the values for the outer diameter in inches and mm, as well as the pitch in threads per inch and also converted into metric measures. Also indicated is the core hole diameter you need when tapping the 20-UN thread. Taps and dies for the 20-UN thread are linked in the table.