The UNJ thread standard (ASME B1.15) defines a system of threads for highly stressed applications requiring high fatigue strength, derived from the military specification MIL-S-8879 used primarily for aerospace fasteners and threaded components. The American Society of Mechanical Engineers developed and published ASME B1.15 in 1995 due to the increase in both use and types of applications. UNJ external threads have a controlled root radius of 0.15011P to 0.18042P, and UNJ internal threads have an increased minor diameter to accommodate the external thread root radius.

What is the UNJ thread form?

UNJ screw threads are of the same form as Unified Screw Threads to ASME/ANSI B1.1 except for specific differences in external and internal threads.

External threads: The root has a maximum and minimum prescribed continuous radius, and is not merely rounded due to tool wear. The controlled root radius is 0.15011P to 0.18042P (MIL-S-8879C).

Internal threads: The minor diameter is increased to accommodate the maximum root radius of the external thread. There is no radius requirement for either the crest or the root of the internal thread.

How are UNJ threads designated?

UNJ product threads are identified by the letter “J” in the thread symbol, and a thread class symbol including an “A” for external threads or a “B” for internal threads.

Can Unified (UN) tooling be used for UNJ threads?

Many of the UNJ thread form characteristics are the same as for UN threads, so some of the tooling used to produce one form can be used to produce the other, but with important restrictions.

External UNJ threads must be produced with a prescribed root radius; therefore, standard Unified Screw Thread (UN) tooling may not be used.

Internal UNJ threads are not required to have a root radius; therefore, ground thread taps designed to produce Unified Screw Threads of the proper class of fit may be used. The letter “J” need not be marked on the tap. The larger product minor diameter of the UNJ internal thread requires the use of a larger tap drill than is used when producing Unified Screw Threads.

What class of fit is a UNJ tap?

UNJ tap is standard 3B class of fit.

Assembly compatibility

UNJ external thread assembles only with UNJ internal thread.

Source and updates

Source: taylortool.com engineering data.