How it connects
Screws onto a matching parallel UNEF male thread. The thread itself does not seal; sealing, if any, comes from a separate gasket, O-ring, flare, compression ferrule or shoulder face.
Male and female halves
Female = internal thread (class designated B, e.g. 2B); male = external thread (class A, e.g. 2A). This connection is the female/internal half.
Usually connects to
- Same-size/pitch external UNEF-2A thread on a fitting or adapter body
Common sizes
- UNEF spans roughly #12 (0.216 in) to 1-11/16 in; pitches include 32, 28, 24, 20 and 18 TPI.
- Examples: 1/4-32, 3/8-32, 1/2-28, 9/16-24, 5/8-24, 7/8-20 and 1-11/16-18. A 9/16-24 nut may be used for 3/8 in tube.
Where it is used
aerospaceinstrumentationthin-wall tubing and fittingselectrical connectors (MIL circular connector couplings)optics and firearms accessoriesHVAC and refrigeration flare fittingsautomotive fluid systemsthin-wall mechanical fasteners and optical/electronic mountings
Variations
- UNEF-1B / 2B / 3B tolerance classes
- UNEFJ / UNR root variants (external)
- 2B tolerance class (standard commercial female thread)
- 3B tolerance class (precision female thread)
How to recognise it
Drawing callout like '5/8-24 UNEF-2B' (diameter-TPI UNEF-class B). Physically a parallel tapped bore with very fine pitch for its diameter; measure major diameter and TPI with a thread gauge and compare to the UNEF table.
Often confused with
| Confused with | How to tell them apart |
|---|---|
| UNF (Unified Fine) thread | Same diameter but fewer threads per inch, e.g. 5/8-18 UNF vs 5/8-24 UNEF; check the TPI in the callout. UNEF has a finer pitch (higher threads per inch) than UNF for the same nominal diameter (e.g., 9/16-24 UNEF vs 9/16-18 UNF). |
| UN constant-pitch series (e.g. 5/8-24 UN) | Identical profile; only the series label differs. Some sizes overlap pitches. |
| NPT / NPS pipe threads | NPT is tapered and has nominal pipe sizes; UNEF is parallel with a diameter-TPI callout. |
| SAE 45° flare / SAE O-ring boss fittings | Those use UNF threads plus a sealing feature; a UNEF callout is a different pitch. |
| NPT / NPTF | UNEF is straight (parallel) and non-sealing on the flanks, whereas NPT is tapered and designed to seal on threads. |
Also called
Unified Extra Fine internal threadNEF thread (older American National Extra Fine)UNEF-2B tapped holeUNEF female threadUNEF nut thread
Good to know
A brass compression reducing coupling may use 5/8-24 UNEF-2B as its body thread. The ferrule makes the tube seal; the UNEF thread is part of that compression assembly.
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