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UNJ Thread Dimensions —
AS8879 Controlled Root Radius Threads

By C&W Engineering Team

UNJ threads are the aerospace version of the unified thread form: same 60° flanks, but with a mandatory controlled root radius of 0.15011P to 0.18042P on the external thread and an enlarged minor diameter on both threads to make room for it. Governed by SAE AS8879 (formerly MIL-S-8879) and ASME B1.15, UNJ is called out on flight fasteners and threaded hardware because the generous root radius removes the sharp-root stress concentration where fatigue cracks start.[1][2]

What Makes UNJ Different from UN

External threads (bolts, studs, machined male threads). The root must carry a continuous radius between 0.15011P and 0.18042P, verified by inspection; it is a controlled feature, not an incidental rounding from tool wear. Because of the larger root radius, the external minor diameter is larger than a UNR or rolled UN root of the same size. Standard UN threading tools cannot produce a conforming J root; J-form inserts or ground tooling are required.[1]

Internal threads (nuts, tapped holes). No root radius is required. Instead, the minor diameter is increased on both threads (the basic thread height drops from 0.625H to 0.5625H, per the 9H/16 thread depth in the specification) so the nut's crest clears the bolt's root radius. A standard class 3B ground tap cuts a conforming UNJ internal thread; what changes is the tap drill, which must be larger than the UN tap drill for the same size.[1][3]

⚠ Interchangeability rule

A UNJ external thread assembles only with a UNJ internal thread. Threading a J-root bolt into a standard UN tapped hole creates interference between the bolt's enlarged root and the nut thread's crest. The reverse is acceptable: a UNJ internal thread clears any standard UN external thread. Never tap a hole with a standard UN minor diameter for a J-callout fastener.[3]

UNJ vs UN thread form, one pitch

Basic profile for one pitch P, drawn to scale (H = 0.866P is the height of the sharp 60° triangle). Both forms share the flanks and the pitch diameter D − 0.6495P. UNJ replaces the flat UN root (truncated 0.25H from the sharp V) with a fillet of 0.15011P to 0.18042P and raises the internal minor diameter from D − 1.0825P (0.625H thread height) to D − 0.9743P (0.5625H, the 9H/16 in AS8879, a 0.3125H truncation) so the nut crest clears that fillet. The grey sliver is the crest material a standard UN nut keeps, which is what lands on a J root. Values are the ones quoted in the text and in footnote [1].

UNJF (Fine Series) Dimensions

Fine pitch is the dominant aerospace series. Basic pitch diameter is identical to the equivalent UNF size; the J-form columns are what change. The suggested drill is the smallest standard drill at or above the basic J minor diameter; treat it as a starting point and verify the finished minor diameter against the AS8879 class 3B limits for the size.[1]

Thread	Basic Pitch Dia (in)	Ext. Root Radius (in)	J Minor Dia, Basic (in)	UN Minor, Ref (in)	Suggested Drill
#4-48 UNJF	0.0985	0.0031–0.0038	0.0917	0.0894	#42 (0.0935)
#6-40 UNJF	0.1218	0.0038–0.0045	0.1136	0.1109	#32 (0.1160)
#8-36 UNJF	0.1460	0.0042–0.0050	0.1369	0.1339	#28 (0.1405)
#10-32 UNJF	0.1697	0.0047–0.0056	0.1596	0.1562	#20 (0.1610)
1/4-28 UNJF	0.2268	0.0054–0.0064	0.2152	0.2113	7/32 (0.2188)
5/16-24 UNJF	0.2854	0.0063–0.0075	0.2719	0.2674	I (0.2720)
3/8-24 UNJF	0.3479	0.0063–0.0075	0.3344	0.3299	R (0.3390)
7/16-20 UNJF	0.4050	0.0075–0.0090	0.3888	0.3834	25/64 (0.3906)
1/2-20 UNJF	0.4675	0.0075–0.0090	0.4513	0.4459	29/64 (0.4531)
9/16-18 UNJF	0.5264	0.0083–0.0100	0.5084	0.5024	33/64 (0.5156)
5/8-18 UNJF	0.5889	0.0083–0.0100	0.5709	0.5649	37/64 (0.5781)
3/4-16 UNJF	0.7094	0.0094–0.0113	0.6891	0.6823	45/64 (0.7031)
7/8-14 UNJF	0.8286	0.0107–0.0129	0.8054	0.7977	13/16 (0.8125)
1-12 UNJF	0.9459	0.0125–0.0150	0.9188	0.9098	59/64 (0.9219)

UNJC (Coarse Series) Dimensions

Thread	Basic Pitch Dia (in)	Ext. Root Radius (in)	J Minor Dia, Basic (in)	UN Minor, Ref (in)	Suggested Drill
#4-40 UNJC	0.0958	0.0038–0.0045	0.0876	0.0849	#43 (0.0890)
#6-32 UNJC	0.1177	0.0047–0.0056	0.1076	0.1042	7/64 (0.1094)
#8-32 UNJC	0.1437	0.0047–0.0056	0.1336	0.1302	#29 (0.1360)
#10-24 UNJC	0.1629	0.0063–0.0075	0.1494	0.1449	#25 (0.1495)
1/4-20 UNJC	0.2175	0.0075–0.0090	0.2013	0.1959	13/64 (0.2031)
5/16-18 UNJC	0.2764	0.0083–0.0100	0.2584	0.2524	G (0.2610)
3/8-16 UNJC	0.3344	0.0094–0.0113	0.3141	0.3073	O (0.3160)
7/16-14 UNJC	0.3911	0.0107–0.0129	0.3679	0.3602	U (0.3680)
1/2-13 UNJC	0.4500	0.0115–0.0139	0.4251	0.4167	7/16 (0.4375)
9/16-12 UNJC	0.5084	0.0125–0.0150	0.4813	0.4723	31/64 (0.4844)
5/8-11 UNJC	0.5660	0.0136–0.0164	0.5364	0.5266	35/64 (0.5469)
3/4-10 UNJC	0.6850	0.0150–0.0180	0.6526	0.6417	21/32 (0.6562)
7/8-9 UNJC	0.8028	0.0167–0.0200	0.7667	0.7547	25/32 (0.7812)
1-8 UNJC	0.9188	0.0188–0.0226	0.8782	0.8647	57/64 (0.8906)

How much larger the J minor is

J minor minus UN minor in inches for every row of the UNJF and UNJC tables above, read from the tables when the page renders. The gap is the 0.625H to 0.5625H change in internal thread height, so it grows with pitch: coarse threads need a noticeably bigger tap drill than fine ones. Hover a bar for both minors.

Why the Root Radius Matters

Fatigue cracks in threaded parts start at the thread root, the sharpest feature under the highest local stress. Replacing a sharp or incidentally-rounded root with a controlled radius of at least 0.15011P substantially lowers the stress concentration factor and with it the crack initiation risk under cyclic load. That is why UNJ is mandatory on most flight-critical fasteners and why rolled threads (which additionally leave compressive residual stress in the root) are preferred for bolts.[2]

On drawings. The J appears in the thread callout: .2500-28 UNJF-3A for an external thread, .2500-28 UNJF-3B for a tapped hole. Class 3 fits are standard; loose classes are not part of AS8879. If a print shows UNJ, quote and machine it as UNJ; substituting UNF silently changes the certified part.[1]

In plain terms

UNJ is the same thread you know with the sharp corner at the bottom replaced by a controlled fillet, plus paperwork to prove it. For tapped holes the practical difference is one drill size: use the J minor column above instead of the standard tap drill chart, and the same 3B tap you already own finishes the job. For external threads it is a tooling and inspection difference, so tell your shop up front that the callout is J-form.

Flight hardware threads, done to the letter

We machine UNJ external threads with J-form tooling and tap J-minor holes daily for aerospace and defense customers, with AS9100D traceability and ITAR registration.

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Sources & References

[1]SAE AS8879 — Screw Threads: UNJ Profile, Inch, Controlled Radius Root with Increased Minor Diameter (supersedes MIL-S-8879). Root radius 0.15011P to 0.18042P; internal thread height 9H/16 (0.5625H). Basic dimensions in the tables computed from the standard's thread-form geometry (pitch diameter D − 0.6495P; basic J minor D − 0.9743P) and checked against the worked example in MIL-S-8879C §3.3.2 (8.750-8 UNJS: computed root radius 0.0188–0.0226, internal minor diameter minimum 8.6283, pitch diameter 8.6688 — all matching the specification).

[2]ASME B1.15 — Unified Inch Screw Threads (UNJ Thread Form). Civil adoption of the UNJ profile for high-fatigue applications.

[3]Taylor Toolworks, "UNJ Profile: Controlled Root Radius with Increased Minor Diameter" technical sheet. Tooling and interchangeability practice: standard 3B taps acceptable, larger tap drill required, UNJ external assembles only with UNJ internal.

UNJ Thread Dimensions —AS8879 Controlled Root Radius Threads