The Unified National Fine (UNF) thread is part of the Unified Thread Standard (UTS) and features a finer pitch compared to Unified National Coarse (UNC) threads. The increased number of threads per inch (TPI) results in a stronger, tighter fit, making UNF threads ideal for applications requiring high-strength fastenings, vibration resistance, and precise engagement.
Key Features of UNF Threads:
✔ Higher thread engagement than UNC, offering increased tensile strength.
✔ Better resistance to loosening due to the finer pitch, making them suitable for vibration-prone environments.
✔ Used in high-precision engineering, such as automotive, aerospace, and hydraulic applications.
✔ Thread angle of 60°, ensuring compatibility with other Unified thread forms.
Common Applications of UNF Threads:
- Automotive industry (engine bolts, suspension components, wheel lugs)
- Aerospace engineering (aircraft fasteners, high-strength assemblies)
- Hydraulic & pneumatic systems (provides better sealing in high-pressure environments)
- Precision tools & machinery (fine threads offer stronger, tighter joints in precision components)
This standard is closely related to:
- UNC (Unified National Coarse) (For applications needing coarser threads with better resistance to stripping.)
- UNEF (Unified National Extra Fine) (For even higher precision and stronger engagement.)
Technical Specifications
UNF threads share the same 60° thread angle as UNC threads, but with a smaller pitch, meaning more threads per inch (TPI). This finer pitch results in a greater clamping force and better resistance to loosening in high-vibration applications.
UNF Thread Profile Parameters
- Thread Angle: 60°
- Crest Rounding: Defined for smooth engagement
- Tolerances: Defined in ANSI B1.1 for external and internal threads
UNF 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.
UNF Thread Dimensions (Inches)
| Nominal Size | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|
| #0 | 0.0600 | 0.0534 | 0.0494 | 0.0465 |
| #1 | 0.0730 | 0.0658 | 0.0616 | 0.0570 |
| #2 | 0.0860 | 0.0776 | 0.0732 | 0.0730 |
| #3 | 0.0990 | 0.0895 | 0.0850 | 0.0890 |
| #4 | 0.1120 | 0.1014 | 0.0969 | 0.0930 |
| #5 | 0.1250 | 0.1121 | 0.1075 | 0.1060 |
| #6 | 0.1380 | 0.1228 | 0.1179 | 0.1200 |
| #8 | 0.1640 | 0.1465 | 0.1412 | 0.1360 |
| #10 | 0.1900 | 0.1702 | 0.1645 | 0.1590 |
| 1/4″ | 0.2500 | 0.2251 | 0.2182 | 0.2130 |
| 5/16″ | 0.3125 | 0.2806 | 0.2729 | 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 |
🔹 Note: The tap drill size is recommended for internal thread cutting.
UNF Thread Dimensions (Metric – mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| #0 | 1.52 | 1.36 | 1.25 | 1.18 |
| #1 | 1.85 | 1.67 | 1.56 | 1.45 |
| #2 | 2.18 | 1.97 | 1.86 | 1.85 |
| #3 | 2.51 | 2.27 | 2.16 | 2.26 |
| #4 | 2.84 | 2.58 | 2.46 | 2.36 |
| #5 | 3.18 | 2.85 | 2.73 | 2.69 |
| #6 | 3.51 | 3.12 | 3.00 | 3.05 |
| #8 | 4.17 | 3.72 | 3.59 | 3.45 |
| #10 | 4.83 | 4.32 | 4.18 | 4.04 |
| 1/4″ | 6.35 | 5.72 | 5.54 | 5.41 |
| 5/16″ | 7.94 | 7.13 | 6.93 | 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 |
🔹 Note: The tap drill size is recommended for internal thread cutting.
Related Inch-Based Thread Standards
- UNC – Unified National Coarse (Coarse pitch threads for general-purpose applications.)
- UNEF – Unified National Extra Fine (High-precision threads for fine 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.