What are "J" threads?

Threads specified with a “J” designation are intended for applications in which high-temperature and high stress levels are experienced and a high fatigue life is required commensurate with the size and weight of the component.

The internal thread is made to a Unified form with the minor diameter modified according to MIL-S-8879 and a 3B tolerance class.

"J" threads are used in aircraft engines and air frames, missiles, space vehicles and similar design areas where size and weight are critical.

The safety-critical nature of “J” series threads leaves no room for quality errors and presents one of the most difficult aerospace metal-cutting operations.