The Unified National Special (UNS) thread is part of the Unified Thread Standard (UTS) but differs from UNC, UNF, and UNEF in that it does not have a standard pitch designation. Instead, UNS threads are custom-designed for specific applications where standard pitches do not provide optimal performance.
Key Features of UNS Threads:
✔ Custom-pitch threads used when standard UNC, UNF, or UNEF threads are unsuitable.
✔ Wide range of applications including automotive, aerospace, medical devices, and custom fasteners.
✔ Ensures precise fits for high-load, high-strength, or specialized engineering needs.
✔ Typically used in unique thread forms such as extra-thin-wall tubing, specialty bolts, and fine-threaded components.
Common Applications of UNS Threads:
- Aerospace & Defense (Custom bolts, structural fasteners, and precision components.)
- Automotive & Motorsport (High-performance and lightweight fasteners.)
- Medical Devices (Implants, orthopedic screws, and specialized surgical equipment.)
- Hydraulic & Pneumatic Systems (Threads requiring extreme sealing properties.)
- Industrial & Custom Machinery (Threads designed for unique mechanical fits and clearances.)
This standard is closely related to:
- UNC (Unified National Coarse) (Standard coarse threads for general applications.)
- UNF (Unified National Fine) (Fine threads for higher strength in limited spaces.)
- UNEF (Unified National Extra Fine) (Precision threads for specialty applications.)
- UNJ (Unified J-Form Threads) (Aerospace-grade threads with enhanced root radius for fatigue resistance.)
Technical Specifications
Unlike UNC, UNF, and UNEF, which have defined thread pitches, UNS threads are customized and chosen based on specific engineering requirements. They maintain the same 60° thread angle and tolerances as other Unified threads but allow custom pitch variations.
UNS Thread Profile Parameters
- Thread Angle: 60°
- Custom Pitch: Varies based on application requirements
- Tolerances: Defined by ANSI B1.1 and application-specific standards
UNS 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 for commonly used UNS threads.
UNS Thread Dimensions (Inches)
| Nominal Size | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|
| #0 | 0.0600 | 0.0550 | 0.0514 | 0.0455 |
| #1 | 0.0730 | 0.0678 | 0.0640 | 0.0560 |
| #2 | 0.0860 | 0.0804 | 0.0764 | 0.0710 |
| #3 | 0.0990 | 0.0925 | 0.0881 | 0.0860 |
| #4 | 0.1120 | 0.1047 | 0.0999 | 0.1000 |
| #5 | 0.1250 | 0.1170 | 0.1122 | 0.1130 |
| #6 | 0.1380 | 0.1285 | 0.1234 | 0.1270 |
| #8 | 0.1640 | 0.1524 | 0.1470 | 0.1520 |
| #10 | 0.1900 | 0.1766 | 0.1719 | 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 |
| 2-1/4″ | 2.2500 | 2.0279 | 2.0146 | 2.0270 |
| 2-1/2″ | 2.5000 | 2.2509 | 2.2370 | 2.2500 |
UNS Thread Dimensions (Metric – mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| #0 | 1.52 | 1.40 | 1.31 | 1.16 |
| #1 | 1.85 | 1.72 | 1.63 | 1.42 |
| #2 | 2.18 | 2.04 | 1.94 | 1.80 |
| #3 | 2.51 | 2.35 | 2.24 | 2.18 |
| #4 | 2.84 | 2.66 | 2.54 | 2.54 |
| #5 | 3.18 | 2.97 | 2.84 | 2.87 |
| #6 | 3.51 | 3.26 | 3.13 | 3.22 |
| #8 | 4.17 | 3.87 | 3.73 | 3.86 |
| #10 | 4.83 | 4.49 | 4.35 | 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 |
| 2-1/4″ | 57.15 | 51.41 | 50.96 | 51.41 |
| 2-1/2″ | 63.50 | 57.00 | 56.53 | 57.00 |
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.)
- UNEF – Unified National Extra Fine (Precision threads for specialty applications.)
- 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.