Guide
What is a UNEF thread, and when should a machined part use it instead of UNF?
A UNEF thread is a Unified Extra Fine inch screw thread: the same 60-degree Unified form as UNC and UNF, but with more threads per inch on each diameter, defined for sizes from #12 to 1-11/16 in ASME B1.1. It exists for parts where the wall under the thread or the available length of engagement is too small for a UNF pitch, such as thin-walled tubes, thin nuts, ferrules and connector couplings.
By Saola Manufacturing engineering team · Published · Updated
Most searches for UNEF end at a size chart. A chart tells you what 1/2-28 UNEF measures; it does not tell you whether your part should carry it. This guide gives both: the full UNEF size table with class 2B minor diameter limits and in-band tap drills, the geometry that makes UNEF the right thread on thin walls, a decision rule against UNF, how a CNC shop actually produces fine pitches, and how to write the callout. The general coarse-or-fine question is covered in the UNC vs UNF guide; this page starts where that one stops.
On this page
How is a UNEF thread different from UNF and UNC?
Only the pitch differs: UNEF uses more threads per inch than UNF on the same nominal diameter, so its threads are shallower. The thread profile, the tolerance classes and the gaging system are the same for all three series.
ASME B1.1 groups Unified inch threads into graded-pitch series (coarse UNC, fine UNF, extra fine UNEF) and constant-pitch series such as 8UN, 12UN, 16UN and 20UN. The federal screw-thread handbook frames the extra-fine series as the choice where even finer pitches than UNF are wanted for short lengths of engagement and for thin-walled tubes, nuts, ferrules or couplings (NBS Handbook H28). Older drawings may show the same series as NEF, the American National designation that the Unified system replaced.
The pitch steps are easy to remember. Per ASME B1.1, a 1/2 in nominal thread is 13 TPI in UNC, 20 in UNF and 28 in UNEF; a 1 in nominal thread is 8, 12 and 20. Because the thread height is a fixed fraction of the pitch (five-eighths of the sharp-V height, per ASME B1.1), each step finer removes less metal from the wall. UNEF and UNF never share a pitch at the same diameter, so a UNEF screw will not enter a UNF hole of the same size, even though the two can look identical at arm’s length.
What are the UNEF thread sizes, pitches and tap drills?
The UNEF series has 25 sizes, from #12-32 to 1-11/16-18, with the pitch stepping from 32 TPI on the smallest sizes to 18 TPI from 1-1/16 upward, per ASME B1.1. The table lists the basic dimensions, the class 2B minor diameter band and a drill that lands inside it.
Pitch diameter and minor diameter in the table are basic values computed from the ASME B1.1 formulas (pitch diameter equals major diameter minus 0.649519 times the pitch; internal minor diameter equals major diameter minus 1.082532 times the pitch), and the 2B maximum minor diameter uses the B1.1 minor diameter tolerance formula, including its lower limit of 0.25P minus 0.4P² on the #12 size. Rounded to three places, every 2B band matches the limit tables in NBS Handbook H28. The drill column is the one detail charts often get wrong. From 9/16-24 upward, no common fractional drill falls inside the class 2B minor diameter band at the 24 and 18 TPI pitches, so a metric drill, a boring pass or a helically interpolated pre-bore is the honest answer. Gage the result, not the drill.
| Size and TPI | Basic major dia (in) | Basic pitch dia (in) | Class 2B minor dia, min–max (in) | Tensile stress area (in²) | Drill inside the 2B minor band (% thread) | UNF pitch at the same size (TPI) |
|---|---|---|---|---|---|---|
| #12-32 UNEF | 0.2160 | 0.1957 | 0.1822–0.1896 | 0.0270 | #13 (76%) | 28 |
| 1/4-32 UNEF | 0.2500 | 0.2297 | 0.2162–0.2236 | 0.0379 | 7/32 (77%) | 28 |
| 5/16-32 UNEF | 0.3125 | 0.2922 | 0.2787–0.2861 | 0.0625 | 9/32 (77%) | 24 |
| 3/8-32 UNEF | 0.3750 | 0.3547 | 0.3412–0.3486 | 0.0932 | 11/32 (77%) | 24 |
| 7/16-28 UNEF | 0.4375 | 0.4143 | 0.3988–0.4073 | 0.1274 | Y (72%) | 20 |
| 1/2-28 UNEF | 0.5000 | 0.4768 | 0.4613–0.4698 | 0.1700 | 15/32 (67%) | 20 |
| 9/16-24 UNEF | 0.5625 | 0.5354 | 0.5174–0.5271 | 0.2139 | 13.3 mm (72%) | 18 |
| 5/8-24 UNEF | 0.6250 | 0.5979 | 0.5799–0.5896 | 0.2682 | 14.8 mm (78%) | 18 |
| 11/16-24 UNEF | 0.6875 | 0.6604 | 0.6424–0.6521 | 0.3287 | 16.4 mm (77%) | none |
| 3/4-20 UNEF | 0.7500 | 0.7175 | 0.6959–0.7074 | 0.3863 | 45/64 (72%) | 16 |
| 13/16-20 UNEF | 0.8125 | 0.7800 | 0.7584–0.7699 | 0.4582 | 49/64 (72%) | none |
| 7/8-20 UNEF | 0.8750 | 0.8425 | 0.8209–0.8324 | 0.5362 | 53/64 (72%) | 14 |
| 15/16-20 UNEF | 0.9375 | 0.9050 | 0.8834–0.8949 | 0.6204 | 57/64 (72%) | none |
| 1-20 UNEF | 1.0000 | 0.9675 | 0.9459–0.9574 | 0.7107 | 61/64 (72%) | 12 |
| 1-1/16-18 UNEF | 1.0625 | 1.0264 | 1.0024–1.0150 | 0.7986 | 25.6 mm (76%) | none |
| 1-1/8-18 UNEF | 1.1250 | 1.0889 | 1.0649–1.0775 | 0.9007 | 27.2 mm (75%) | 12 |
| 1-3/16-18 UNEF | 1.1875 | 1.1514 | 1.1274–1.1400 | 1.0089 | 28.8 mm (74%) | none |
| 1-1/4-18 UNEF | 1.2500 | 1.2139 | 1.1899–1.2025 | 1.1232 | 30.4 mm (74%) | 12 |
| 1-5/16-18 UNEF | 1.3125 | 1.2764 | 1.2524–1.2650 | 1.2437 | 32.0 mm (73%) | none |
| 1-3/8-18 UNEF | 1.3750 | 1.3389 | 1.3149–1.3275 | 1.3703 | 33.6 mm (72%) | 12 |
| 1-7/16-18 UNEF | 1.4375 | 1.4014 | 1.3774–1.3900 | 1.5030 | 35.1 mm (77%) | none |
| 1-1/2-18 UNEF | 1.5000 | 1.4639 | 1.4399–1.4525 | 1.6419 | 36.7 mm (76%) | 12 |
| 1-9/16-18 UNEF | 1.5625 | 1.5264 | 1.5024–1.5150 | 1.7869 | 38.3 mm (76%) | none |
| 1-5/8-18 UNEF | 1.6250 | 1.5889 | 1.5649–1.5775 | 1.9381 | 39.9 mm (75%) | none |
| 1-11/16-18 UNEF | 1.6875 | 1.6514 | 1.6274–1.6400 | 2.0954 | 41.5 mm (74%) | none |
Why do connector bodies, thin-wall tubes and jam nuts use UNEF threads?
Because a finer pitch cuts a shallower thread, it leaves more metal under the thread root and fits more full threads into a short length. On a thin tube or a short coupling nut, that is the difference between a sound thread and a wall that cracks or a joint with too few threads to align.
Take a tube with a 1 in outside diameter bored to 0.875 in and use the class 2A limit tables in NBS Handbook H28: a coarse 1-8 UNC thread would cut below the bore and break through; a 1-12 UNF thread leaves about a third of the wall that UNEF leaves; and 1-20 UNEF leaves a usable ring of metal under the root. The table below works the same tube through all three series. The same logic explains why circular connector coupling rings, panel-mount bulkhead nuts and other thin retaining rings often carry UNEF pitches: the threaded section is a short, thin ring, and the thread exists mainly to locate and clamp, not to carry a large bolt load.
| Thread | Pitch (in) | Basic thread height (in) | External minor dia, 2A UNR max (in) | Wall left under the root (in) | Full threads in 0.25 in of length |
|---|---|---|---|---|---|
| 1-8 UNC | 0.1250 | 0.0677 | 0.8446 | none (breaks into the bore) | 2 |
| 1-12 UNF | 0.0833 | 0.0451 | 0.8960 | 0.0105 | 3 |
| 1-20 UNEF | 0.0500 | 0.0271 | 0.9373 | 0.0312 | 5 |
Bearing locknuts are a common point of confusion. Inch-series bearing locknuts and their shaft threads often follow the bearing maker’s own fine-pitch special thread rather than a UNEF size, so copy the shaft thread from the locknut catalog and designate it as a UNS or constant-pitch UN thread instead of rounding it to the nearest UNEF entry.
When should a machined part use UNEF instead of UNF?
Use UNEF only when a specific constraint forces it: a mating part or connector that is already UNEF, a wall too thin to keep sound metal under a UNF root, or a length too short to hold enough UNF threads. Otherwise UNF or UNC is the better default.
The fine pitch is not free. The thread is shallower, so a burr, a dent, a plating build-up or a slightly oversize minor diameter takes a bigger share of the flank. It cross-threads more easily on assembly, it is less forgiving in soft aluminum and in die-cast or other low-ductility metal, and UNEF taps and gages are less often held in stock than UNF. A finer pitch also buys no stripping strength in a weak parent metal: the tapped-hole pull-out formula in NASA’s fastener design manual depends on the pitch diameter, the shear strength of the hole material and the engagement length, with no pitch term, so a UNEF hole in aluminum or cast metal needs as much engagement as a coarser thread of the same size (NASA RP-1228). The thread engagement guide shows how to check the length.
| Condition on the part | Thread to specify | What to write or check |
|---|---|---|
| Mates with a purchased connector, fitting, coupling or nut that is UNEF | UNEF, same size and pitch | Copy the mating part’s callout and class exactly |
| Wall under a UNF root would be thinner than about one UNF thread height | UNEF | Compute wall from the external or internal minor diameter limits, not the basic size |
| Engagement length holds fewer than about 4 full UNF threads, and the joint mainly locates or clamps | UNEF | Check stripping load if the joint carries real axial load |
| Tapped hole in aluminum, zinc or another soft or cast metal, ordinary wall | UNC, or UNF if vibration requires | See the coarse vs fine guide; UNEF adds risk with no gain |
| Joint opened often, or threads plated or anodized | UNF or UNC | Coating thickness eats a larger share of a fine pitch’s allowance |
| Fine pitch needed on a size or diameter not in the UNEF list | Constant-pitch UN (for example 16UN or 20UN) or UNS | Write the pitch and series explicitly; do not label it UNEF |
Should UNEF threads be tapped, thread milled or single-point turned?
External UNEF threads and large internal ones are usually single-point turned or thread milled, because one tool covers every diameter at a given pitch. Tapping makes sense for small internal UNEF sizes at production volume, where a stock tap exists.
On a lathe the pitch is just a programmed lead, so a coupling ring or a fitting body with a 3/4-20 UNEF outside thread costs about the same to single-point as a UNF thread. Internal threads are where the choice shows. A small 28 TPI single-profile thread mill cuts 7/16-28 UNEF, 1/2-28 UNEF and 1/4-28 UNF with the same tool, while each of those needs its own tap. Large UNEF taps are specialty items, and the fine flank is unforgiving of a tap that wanders or a chip that packs. The trade-offs, including when tapping still wins, are covered in thread milling vs tapping.
We review fine-pitch threads on a drawing before quoting: whether the part is a turned body that can be single-pointed in the same setup on CNC turning, whether the internal thread is better interpolated on CNC milling, and whether the mating part is a purchased connector or fitting whose maker’s callout should govern. When a UNEF thread appears on a soft-metal part with an ordinary wall, we ask whether the pitch was intended, since changing it early is cheaper than special gages later. Drawings can be sent through the quote form.
How do you call out a UNEF thread on a drawing?
Write nominal size, threads per inch, series and class in that order, for example 1/2-28 UNEF-2B for an internal thread or 3/4-20 UNEF-2A for an external one, then add LH, depth or length as needed. Use class 2A and 2B unless the mating part requires 3A or 3B.
The designation format is the same as for any Unified thread per ASME B1.1. A tapped hole reads
4X 1/2-28 UNEF-2B ↓ .50 MIN FULL THREAD; a turned external thread reads
3/4-20 UNEF-2A with the thread length dimensioned on the view. Add -LH for a
left-hand thread and UNREF only when the external root radius is required. Class 2A/2B is the
general-purpose fit and the right default; 3A/3B tightens the pitch diameter tolerance and removes the 2A
allowance, which helps a precision coupling but not stripping strength, as explained in
class 2B vs 3B. The full callout grammar, including depth symbols,
is in the thread callout guide.
On the RFQ, say what the thread mates with. If it is a purchased connector, fitting or locknut, give its part number so the supplier gages to the right class, and state whether threads are measured before or after any plating or anodize. If inspection should use specific gages, name them; UNEF go/no-go plug and ring gages are less common than UNF, and the supplier should confirm they are available rather than discovering the gap at first article.
Frequently asked
Are UNF and UNEF threads the same?
No. They share the Unified 60-degree profile and the same tolerance classes, but at every diameter where both exist UNEF has more threads per inch, so the parts do not interchange. A 1/2-20 UNF screw will not thread into a 1/2-28 UNEF hole.
What does UNEF stand for?
Unified National Extra Fine. The “UN” marks the Unified thread form shared by the US, UK and Canada; “EF” marks the extra-fine graded-pitch series. UNREF is the same series with a mandatory rounded root on the external thread.
Is a UNEF thread a pipe thread or a sealing thread?
No. UNEF is a straight thread, so it does not seal on the flanks the way a tapered NPT thread does. Fittings that use UNEF or other straight threads seal with an O-ring, gasket or face seal, and the thread only holds the joint together.
What is the difference between UNEF and UNJEF?
UNJEF uses the same sizes and pitches but the UNJ form, with a larger controlled root radius on the external thread and a correspondingly larger minimum minor diameter on the internal thread, to improve fatigue life. It is common in aerospace hardware; a UNJ external thread should not be assumed to fit an ordinary UN internal thread without checking the minor diameter.
Can a UNEF thread be specified in metric terms?
There is no metric equivalent that interchanges. The nearest fine ISO metric pitches are close but not equal, so a metric part must be redesigned rather than relabeled. If the mating hardware is inch, keep the UNEF callout and dimension the rest of the part as needed.
Sources
- ASME B1.1, Unified Inch Screw Threads (UN, UNR and UNJ Thread Forms), ASME, 2024 edition
- Screw-Thread Standards for Federal Services, Handbook H28, Part I, National Bureau of Standards, 1969
- Fastener Design Manual, NASA RP-1228, NASA Lewis Research Center, 1990
- ASME B1.2, Gages and Gaging for Unified Inch Screw Threads, ASME, accessed 2026