UNJ threads are controlled root radius threads. Normally the radius on the crest of the internal thread form is not controlled but in UNJ applications it is. I machined some rocket motor cases with this thread form many years ago.
There is only two ways to inspect this feature on an internal thread either sectioning the part and using a optical comparator or using some expansion foam or clay and filling the thread form and removing the resultant form and checking the root radius.
I assume this thread form is used to reduce the stress riser at the root of the thread. I personally do not think this thread form has much impact on the servicability of the part but if its on the print you should try to conform.
If the part is manufactured on a lathe the threading tool will need to have the proper radius however, if this is tapped, a special tap will be required.
It is an all-too-common misconception that UNJ internal threads have a radiused crest. They do not.
The "J" thread form is a modified thread form with a controlled, tangent root radius on the external thread, and an increased minor diameter on external and internal threads. It is usually specified in aerospace of in fatigue critical applications.
The sole modification to the internal thread form/dimensions (when compared to a “standard”, “non-J” thread) is a small increase in the thread minor diameter (the “pre-tap” or “pre-thread-cutting” diameter) to prevent interference of the radiused external thread root with the internal thread.
Example: The minor diameter of your 1.125-18 UNJEF-3B thread should be 1.0784/1.0709, where the minor diameter of a 1.125-18 UNEF-3B thread is 1.0730/1.0650.
UNJ internal threads can be inspected with standard threaded plug gages (of the appropriate class). The (modifed) minor diameter is inspected with a gage of the apporpriate size, or directly measured to the applicable requirements.
Reading this brought up a question of mine , Can anyone tell me what the Root Radius of a 7/16-24 UNJS 3A thread is ? We can't find anything in the Machinery Handbook about this and from time to time we have some oddball fittings we have to make and we just roll the dice between a 20 and 28 thread .054 to .075 is that the right thing to do ?
AS8879 provides the formulas for calculating "special" (Non-Standard) UNJ thread dimensions, including root radius and minor diamter dimensions (you'll undoubtedly want both). These calculation are the same as in Table VIII of the now cancelled MIL-S-8879 document (avaiable for free at
