Originally established from a military specification in 1965, J threads are a variation on common UN and metric thread types.

The critical difference between the standard variant of thread to their respective UNJC, UNJF and Metric J (MJ) thread types is that an external J thread has a continuous, rounded root/minor diameter profile, where as a UN and metric thread types are more square.

What are the advantages?

The primary reason for choosing a J thread is, the rounded root/minor diameter profile reduces the stress raisers created by sharp edges or corners, which can break during harsh vibration cycles, fatigue loading or in environments which see large/frequent temperature fluctuations. For these reasons J threads are most commonly used in the aerospace industry.

Compatibility!!!

Internal J threads are almost identical to their respective UN or metric thread types except that the minor diameter of a J thread is slightly larger than the UN or metric minor diameter. This is to allow the minor diameter of the internal thread to clear large root radius of the J external thread.

External J threads, as we now know, have a continuous, rounded root/minor diameter profile which as a result increases the minor diameter of the thread. This is where we can start to run into trouble!

UN or Metric Bolt into UN or Metric Nut

😄 No problems

UNJ or MJ Bolt into UNJ or MJ Nut

😄 No problems

UN or MJ Bolt into UNJ or MJ Nut

🥴 No assembly problems, however the maximum loading strength of the connection will be lower as a result.

UNJ or MJ Bolt into UN or Metric Nut

😩 In most cases, these thread types will assembly, with some brute force, as an interference fit at the thread minor diameter. However this assembly will likely damage both the male and female sides of the threaded connection with the increased risk of reducing the strength of the material and/or the maximum loading capacity of the thread, plus the potential for stress cracking.

Ultimately these two thread types are not designed to be used together!