UNJ and MJ threads, often referred to as “J” threads, are predominately used in the aerospace industry and other applications requiring high fatigue strength, including some automotive applications.  The “UNJ” designation is used for inch screw threads and “MJ” for metric threads.

“J” threads are similar to standard Unified or metric threads except they have a large radius in the root or minor diameter of the external thread.  This radius eliminates sharp corners in the minor diameter of the bolt to increase the stripping strength.  This technique is based upon the principle of using filets or radii in sharp corners on parts to strengthen stress points were cracking or failure may occur due to changes in temperature, vibration, or heavy loads.  All other dimensions, such as pitch diameter and major diameter, are the same as standard threads.

Since the radius on the external thread increases the minor diameter of the bolt, the internal thread or nut must be modified to allow it to assemble.  The minor diameter of the internal thread must be enlarged to clear the radius.  This is the only change to the internal thread.  All other dimension are the same as standard Unified and metric threads.

What type of tools are used to produce “J” threads?  For the internal threads, since the only change is the minor diameter size, only the tap drill size will need to be changed.  The same tap used to produce standard threads is used to produce “J” threads.  All tooling for external threads, such as dies, thread rolls, and chasers, must be made to produce a radius at the minor diameter.  In addition, all run-out or incomplete threads must also have a radius.  In other words, the chamfered teeth on dies and chasers must have rounded crests.  Unfortunately, rounded chamfered teeth are difficult to produce, and therefore, not readily available on dies and chasers.  In order to use these tools to produce “J” threads on a part, an undercut or clearance must be incorporated beyond the full thread requirement to accommodate the chamfer.
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