threaded fastener series

Compare UN, UNR

Index of thread series

The UNJ thread series differs from the UN and UNR in the shape of the root, the bottom of the groove between threads.  In the UNR series, this groove was rounded. UNJ carries this even further, so far that the minor diameter of the bolt is increased. (Imagine a cylinder on which the bottoms of the threads lie. The diameter of that cylinder is the minor diameter.) Because of this, UN nuts will not fit UNJ bolts. Unlike the UNR thread series, which does not define a nut, the UNJ series includes specifications for internal threading, as of nuts.

Gaging issues

The UNJ GO ring gage accomodates a larger minor diameter than the UN GO gage; a UN GO ring gage will reject a UNJ bolt. The GO ring gage is the only gage in which the UNJ model differs from the UN model. A UNJ NOGO ring gauge is the same as a UN NOGO gage; only the marking is different. Similarly, a UN work plug gage will pass a UNJ internal thread. The UN and UNJ set plug gages are also identical; they cannot be used to check the minor diameter of a UNJ GO ring gage, which should be done as a separate operation with plain pin gages after checking the ring gage with a set plug gage. 


UNJC is called “Unified National Coarse thread series with external thread controlled root radius”, and similarly for the other J series.

ISO 3161 Aerospace - UNJ Threads - General Requirements and Limit Dimensions (Dec. 1999)

ANSI/ASME B1.15 - 1995 Unified Inch Screw Threads, UNJ Thread Form and Errata.

MIL-S-8879C, 25 July 1991, (+ Amendment 1, 2 September 1992) Screw Threads, Controlled Radius Root with Increased Minor Diameter, General Specification For.

This Mil Spec was cancelled on 24 Feb 2003, superceded by the SAE standard referenced below. The Department of Defense formally adopted the SAE standard on 14 May 2003. The Federal Aviation Authority also switched (FAA Advisory Circular AC No. :21-41A, 6 June 2003). In all cases, Application Category “Other Thread” in the Mil Spec became “Category 1” in AS8879, and Application Category “Safety Critical Thread” became “Category 2”.

SAE-AS8879, Screw Threads - UNJ Profile, Inch Controlled Radius Root with Increased Minor Diameter.

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