Threads, Threads, & Threads!?

Recently we’ve received inquires regarding the use of UNJ male-threaded fasteners with KATO CoilThread® inserts. However, before we answer the question, a brief explanation of J-threads may be necessary. The difference between UNC/UNF threads and UNJ threads is that the minor diameter of J-threads is curved not squared (see Figure 1).


Figure 1

J-threads are preferred in the Aerospace industry because the curved diameter minimizes stress points created by sharp edges or corners, which can break during vibration.

To answer the million dollar question…Yes, CoilThread inserts will accommodate UNJ, MIL-S-8879, male threaded fasteners.

Why do J-threads work with CoilThread inserts? I was hoping you’d ask that question. When the round wire is rolled to form the diamond shaped cross section per the NAS/MS specifications, the actual form of the wire is curved not flat. Figure 2 shows the theoretical profile versus the actual profile of the wire cross-section.

The curved profile of the inserts wire cross-section will easily accommodate the curved minor diameter of the male J-thread.


Figure 2

If you have any questions, please contact us.