ASME B1.1 – Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms)

Defines thread specifications, including UNC (Unified National Coarse) and UN8 (8 threads per inch), commonly used for stud bolts.

Download the ASME B1.1 pdf standard here

ASME B1.1 – Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms)

This Standard specifies the thread form, series, class, allowance, tolerance, and designation for unified screw threads. (In order to emphasize that unified screw threads are based on inch modules, they may be denoted unified inch screw threads.) Several variations in thread form have been developed for unified threads; however, this Standard covers only [Unified] UN and UNR [external threads only] thread forms. For easy reference, a metric translation of this Standard has been incorporated as Nonmandatory Appendix C. Nonmandatory Appendices A through E contain useful information that is supplementary to the sections of this Standard.

B1.15 – 1995, Unified Inch Screw Threads, UNJ Thread Form has been Incorporated into B1.1 – 2019, Unified Inch Screw Threads (UN, UNR, and UNJ Thread Forms).

The UNR thread form and UN thread form are identical except that the roots of the external threads are rounded with a radius of curvature between 0.108 and 0.144 times the pitch. The rounded root improves fatigue strength compared to the UN thread form.
The UNR thread designation standard was developed during the 1950s due to the request for fatigue-resistant safety- critical fasteners. Almost all externally threaded fastener manufacturers produce UNR rolled threads rather than plain unified form threads.

•     UNC – Unified National Coarse Thread – comparable with the ISO metric thread
•     UNF – Unified National Fine Thread
•     Female and Male UNC/UNF threads are both parallel
•     The UNC/UNF flank angle is 60°
Unified threads comes in three different classes:
a )   for applications where a liberal tolerance is required to permit easy assembly even withslightly nicked threads
b )   most commonly used class for general applications
c )   for applications where closeness of fit and/or accuracy of thread elements are important

UNC threads according ANSI B1.1:

Example – Typical designation of an UNC thread
•    1/4” x 20 UNC

UNF threads ANSI B1.1:

Example – Typical designation of an UNF thread
•    1/2” x 20 UNF