UNJ vs. UN
UNJS vs. UNS
UNJC vs. UNC
UNJF vs. UNF
UNJEF vs. UNEF

WHERE
The J series thread is defined in specification SAE AS8879 (formerly MIL-S-8879)

WHY
The rounded root of a UNJ external thread reduces the rate of threading tool crest wear and greatly improves fatigue strength over that of a flat root UN thread.

INTERNAL SCREW THREAD
The UNJ for internal threads is identical to the UN thread except that the UNJ minor diameter is slightly larger than the UN minor diameter. This is to allow the minor diameter of the internal thread to clear large root radius of the UNJ external thread. If a UNJ bolt is screwed into a UN nut there will in most cases be an interference fit at the thread minor diameter.

EXTERNAL SCREW THREAD
Both UNJ and UN are used for external threads.

External UN threads per ANSI/ASME B1.1
Paragraph 1.3: UN threads have "a flat or optional rounded root contour" at the minor diameter.
Paragraph 2.3.1: UN threads have "a flat root contour" at the minor diameter, but "a rounded root contour cleared beyond the 0.25P flat width of the basic profile is optional"..."to provide for some threading tool crest wear."

External UNJ threads per SAE AS8879 have a rounded controlled root radius at the minor diameter. The minimum root radius is calculated at 0.15011p and the maximum root radius is calculated at 0.18042p. The root contour shall have a smooth, continuous, nonreversing contour and shall blend tangentially into the flanks and any straight segment. The radius is larger than the UNR style.

PRODUCT COMPATIBILITY
UN Bolt into UN Nut = No problems.
UNJ Bolt into UNJ Nut = No problems.
UN Bolt into UNJ Nut = No assembly problems. Functional problems are possible.
UNJ Bolt into UN Nut = In most cases an interference fit at the thread minor diameter. Functional problems are also possible.

SCREW THREAD WORK PLUG GAGES
The only difference between the internal UNJ thread and the standard UN threaded is that the UNJ thread has a larger minor diameter. The thread work plug gage does not check the minor diameter of the thread, thus when checking the threads, standard UN style Work Plug Gages are used. If UNJ gages are ordered from a gage maker, the gage maker will supply the standard UN gage members with a handle marked UNJ. The minor diameter of internal threads should be checked with plain pin gages.

SCREW THREAD RING GAGES
Because of the large root radius on the minor diameter of the external thread, the UNJ GO ring gage is supplied with an enlarged minor designed to clear the root radius. The GO ring gage is the only gage that is modified specifically for the UNJ specification and the only difference between the UNJ GO and the UN GO thread gages is the minor diameter. The UN GO ring gage if used to measure the UNJ thread the gage will reject the part because of the minor diameter interference. The UNJ NOGO ring gage is identical to the UN NOGO ring gage except that it may be marked with a "J". The minor diameter of the NOGO ring gage is relieved to just below the pitch diameter, and because of this the standard UN NOGO gage will clear the minor diameter of both the UN and UNJ external threads.

SCREW THREAD SET PLUG GAGES
The set plug members used for the UNJ series threads are identical to the set plugs used for the UN series. If UNJ gages are ordered from a gage maker, the gage maker will supply the standard UN gage members with a handle marked UNJ. This is because set plug gages do not check the minor diameter of the ring gage they are setting. The minor diameter of the ring gage should be checked with plain pin gages after the ring has been properly set with the set plug.

HOW TO CHECK THE ROOT RADIUS
Use an optical comparator to check the UNJ external thread form root radius at the minor diameter.

THREAD NOMENCLATURE
UN = Basic Unified National thread series
UNJ = Basic Unified National thread series with external thread controlled root radius
UNS = Special Unified National thread series
UNJS = Special Unified National thread series with external thread controlled root radius
UNC = Unified National Coarse thread series
UNJC = Unified National Coarse thread series with external thread controlled root radius
UNF = Unified National Fine thread series
UNJF = Unified National Fine thread series with external thread controlled root radius
UNEF = Unified National Extra Fine thread series
UNJEF = Unified National Extra Fine thread series with external thread controlled root radius

COMMENTS
This data accurate as of March 2008.
Refer comments/corrections to gageguy@gagecrib.com.