The Unified National Extra Fine (UNEF) thread is part of the Unified Thread Standard (UTS) and features an even finer pitch than Unified National Fine (UNF) threads. This results in superior thread engagement, greater tensile strength, and enhanced resistance to loosening in high-vibration environments.
Key Features of UNEF Threads:
✔ Extremely fine thread pitch provides high clamping force and precision.
✔ Greater thread engagement than UNF, making it ideal for thin-walled materials.
✔ Stronger and more vibration-resistant than both UNC and UNF.
✔ Used in critical applications, such as aerospace, precision instrumentation, and medical devices.
Common Applications of UNEF Threads:
- Aerospace & Aviation (fasteners in aircraft frames, turbine engines, and satellite components.)
- Medical Devices & Instruments (high-precision equipment requiring fine adjustments.)
- Precision Machinery & Optics (microscopes, cameras, and fine adjustment mechanisms.)
- Automotive & Motorsport (high-performance engine components and lightweight materials.)
- Hydraulic & Pneumatic Fittings (connections that require high sealing efficiency in thin-walled materials.)
This standard is closely related to:
- UNC (Unified National Coarse) (For general-purpose coarse threads with better resistance to stripping.)
- UNF (Unified National Fine) (For applications needing high strength with standard fine threads.)
- UNS (Unified National Special) (For non-standard pitch threads used in specialized applications.)
Technical Specifications
UNEF threads share the same 60° thread angle as UNC and UNF threads, but have a much finer pitch for increased load-bearing capacity and improved resistance to loosening in high-stress environments.
UNEF Thread Profile Parameters
- Thread Angle: 60°
- Crest Rounding: Defined for smooth engagement
- Tolerances: Defined in ANSI B1.1 for external and internal threads
UNEF Standard Thread Sizes & Dimensions
The following tables provide both inch and metric equivalents for major, pitch, and minor diameters, as well as recommended tap drill sizes.
UNEF Thread Dimensions (Inches)
| Nominal Size | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|
| #0 | 0.0600 | 0.0540 | 0.0504 | 0.0455 |
| #1 | 0.0730 | 0.0668 | 0.0630 | 0.0560 |
| #2 | 0.0860 | 0.0794 | 0.0754 | 0.0710 |
| #3 | 0.0990 | 0.0915 | 0.0871 | 0.0860 |
| #4 | 0.1120 | 0.1037 | 0.0989 | 0.1000 |
| #5 | 0.1250 | 0.1160 | 0.1112 | 0.1130 |
| #6 | 0.1380 | 0.1275 | 0.1224 | 0.1270 |
| #8 | 0.1640 | 0.1514 | 0.1460 | 0.1520 |
| #10 | 0.1900 | 0.1766 | 0.1709 | 0.1750 |
| 1/4″ | 0.2500 | 0.2279 | 0.2214 | 0.2130 |
| 5/16″ | 0.3125 | 0.2806 | 0.2739 | 0.2670 |
| 3/8″ | 0.3750 | 0.3431 | 0.3352 | 0.3230 |
| 7/16″ | 0.4375 | 0.3926 | 0.3839 | 0.3740 |
| 1/2″ | 0.5000 | 0.4552 | 0.4465 | 0.4360 |
| 9/16″ | 0.5625 | 0.5127 | 0.5034 | 0.4950 |
| 5/8″ | 0.6250 | 0.5752 | 0.5660 | 0.5580 |
| 3/4″ | 0.7500 | 0.6901 | 0.6800 | 0.6870 |
| 7/8″ | 0.8750 | 0.8016 | 0.7914 | 0.8020 |
| 1″ | 1.0000 | 0.9129 | 0.9026 | 0.9160 |
| 1-1/8″ | 1.1250 | 1.0244 | 1.0138 | 1.0270 |
| 1-1/4″ | 1.2500 | 1.1359 | 1.1250 | 1.1380 |
| 1-3/8″ | 1.3750 | 1.2474 | 1.2362 | 1.2490 |
| 1-1/2″ | 1.5000 | 1.3589 | 1.3474 | 1.3600 |
| 1-5/8″ | 1.6250 | 1.4704 | 1.4586 | 1.4710 |
| 1-3/4″ | 1.7500 | 1.5819 | 1.5698 | 1.5820 |
| 2″ | 2.0000 | 1.8049 | 1.7922 | 1.8040 |
UNEF Thread Dimensions (Metric – mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| #0 | 1.52 | 1.37 | 1.28 | 1.16 |
| #1 | 1.85 | 1.70 | 1.60 | 1.42 |
| #2 | 2.18 | 2.02 | 1.92 | 1.80 |
| #3 | 2.51 | 2.32 | 2.21 | 2.18 |
| #4 | 2.84 | 2.63 | 2.51 | 2.54 |
| #5 | 3.18 | 2.95 | 2.82 | 2.87 |
| #6 | 3.51 | 3.24 | 3.11 | 3.22 |
| #8 | 4.17 | 3.85 | 3.71 | 3.86 |
| #10 | 4.83 | 4.49 | 4.34 | 4.45 |
| 1/4″ | 6.35 | 5.79 | 5.62 | 5.41 |
| 5/16″ | 7.94 | 7.13 | 6.96 | 6.78 |
| 3/8″ | 9.53 | 8.72 | 8.51 | 8.21 |
| 7/16″ | 11.11 | 9.97 | 9.75 | 9.50 |
| 1/2″ | 12.70 | 11.57 | 11.34 | 11.07 |
| 9/16″ | 14.29 | 13.02 | 12.78 | 12.57 |
| 5/8″ | 15.88 | 14.61 | 14.36 | 14.17 |
| 3/4″ | 19.05 | 17.53 | 17.27 | 17.46 |
| 7/8″ | 22.23 | 20.37 | 20.10 | 20.37 |
| 1″ | 25.40 | 23.19 | 22.91 | 23.27 |
| 1-1/8″ | 28.58 | 26.03 | 25.73 | 26.09 |
| 1-1/4″ | 31.75 | 28.87 | 28.55 | 28.92 |
| 1-3/8″ | 34.93 | 31.71 | 31.36 | 31.75 |
| 1-1/2″ | 38.10 | 34.55 | 34.18 | 34.58 |
| 1-5/8″ | 41.28 | 37.39 | 37.00 | 37.40 |
| 1-3/4″ | 44.45 | 40.23 | 39.82 | 40.23 |
| 2″ | 50.80 | 45.82 | 45.39 | 45.82 |
Related Inch-Based Thread Standards
- UNC – Unified National Coarse (Coarse pitch threads for general-purpose applications.)
- UNF – Unified National Fine (High-strength fine threads for precision applications.)
- UNS – Unified National Special (Non-standard pitch sizes for specialized uses.)
- UNJ – Unified J-Form Threads (Modified root radius for aerospace applications.)
- BSW – British Standard Whitworth (Traditional UK coarse thread system.)
Important Notice on Standards Validity
This information is for reference purposes only. Users should consult official sources and updated standards to ensure compliance with current regulations. The validity of standards may change over time, so it is recommended to verify them through recognized engineering and standardization organizations (ANSI, ASME, ISO, etc.).
Check with relevant standardization bodies in your region to confirm whether a specific standard is still valid and applicable. Regulations and industry requirements may vary based on region and application.