What's The “J” In UNJ Screw Threads?

UNJ, UNJC, UNJF and UNJEF threads are almost identical to UN, UNC, UNF, and UNEF threads except that the external J thread has a much larger root radius than the standard UN threads and inspection must be performed on this element. The Special UNJ threads are designated by the nomenclature UNJS. J screw threads feature a root radius which improves the tensile stress area of the fastener and helps to reduce the stress concentration factor in the thread thus making the thread stronger. Additionally, the requirement for high strength is achieved with 3A and 3B classes of fit. All pitch diameters and tolerances are based on the Unified Inch Standard.

SAE AS8879 was introduced to provide an alternative to the inactive government specification Mil-S-8879 for screw threads – UNJ profile, inches.

Per SAE AS8879, UNJ Thread profiles are not interchangeable with Metric system MJ profiles or UN profile inch screws. The internal UNJ thread will assemble with the standard UN external thread however assembling external UNJ threads with UN series internal threads should be avoided due to potential interference at the minor diameter.

External threads are of Unified form per ASME B1.1 and modified at the root so that the flanks of the adjacent threads are joined by one continuous, smooth and blended curve tangent to the flanks. The radius is specified in the applicable standard.

Internal threads are also of Unified form per ASME B.1. and modified at the minor diameter to the values specified in the applicable standard.

Standard working Go/No Go thread plug gages are used to inspect internal J threads. The gage is typically marked without any modifications to the Plugs.

For external J threads, the Go member is modified to clear the minor diameter. A standard No Go thread ring gage may be used to inspect J threads. Both gages are typically marked accordingly.

Applicable Standards:

ASME B1.1 Unified Inch Screw Threads ( UN and UNR Thread Form )
ASME B1.2 Gages and Gaging for Unified Inch Screw Threads
ASME B1.3 Screw Thread Gaging System for Dimensional Acceptability – Inch and Metric Screw Threads
( UN, UNR, UNJ, M and MJ
ASME B1.7 Nomenclature, Definitions, and Letter Symbols for Screw Threads
ASME B46.1 Surface Texture ( Surface Roughness, Waviness and Lay )
ASME B47.1 Gage Blanks