UN and UNR are the standard codes of the unified thread in the United States. UNR stipulates that the profile of the thread root must be a smooth arc connection, while the root of the UN thread is flat or randomly rounded.
UN can also be subdivided into: UNC (coarse pitch series), UNF (fine pitch series), UNEF (ultra-fine pitch series) and UNS (special pitch series). , 6UN, 8UN, 12UN, 16UN, 20UN, 28UN and 32UN, the number is the number of teeth per inch, and then only two letters UN are marked, but if the pitch of a large diameter specification is the same as the constant pitch, it has been included in UNC, UNF Or the UNEF regulations, it should be marked with UNC, UNF and UNEF more in line with the standard, so 15/16-20UN-2A, strictly speaking, should be 15/16-20UNEF-2A.
If the major diameter and pitch meet the requirements of the UN standard, it cannot be said to be "non-standard", at most it can only be said to be "extraordinary specification", and 15/16-20UNEF-2A is also
If there are standards, it cannot be said to be "non standard".