UNC (Unified National Coarse) threads are part of the Unified Thread Standard (UTS), primarily used in inch-based fastening systems. These threads have a coarse pitch, making them stronger, more durable, and resistant to wear compared to fine threads.
UNC threads are commonly used in:
- General-purpose fasteners (bolts, screws, nuts)
- Structural applications (construction, automotive, and heavy machinery)
- Soft material fastening (aluminum, plastics, and wood applications)
This standard is closely related to:
- UNF (Unified National Fine) (For applications needing better grip and higher thread engagement.)
- UNEF (Unified National Extra Fine) (For high-precision applications.)
Technical Details
UNC threads are characterized by:
✅ A 60° thread angle (standardized for strength and compatibility)
✅ Greater thread depth (stronger engagement in soft materials)
✅ Better resistance to stripping and loosening
✅ Faster assembly due to coarser pitch spacing
UNC Thread Profile Parameters
- Thread Angle: 60°
- Crest Rounding: Defined for smooth engagement
- Tolerances: Defined in ANSI B1.1 for external and internal threads
UNC Standard Thread Sizes & Dimensions
Below is an extended table covering a wider range of standard UNC thread sizes.
Standard UNC Threads – Major, Minor, and Pitch Diameters
| Nominal Size (inches) | Threads Per Inch (TPI) | Major Diameter (in) | Pitch Diameter (in) | Minor Diameter (in) | Tap Drill Size (in) |
|---|---|---|---|---|---|
| #0 | 80 | 0.0600 | 0.0513 | 0.0457 | 0.0465 |
| #1 | 64 | 0.0730 | 0.0629 | 0.0561 | 0.0570 |
| #2 | 56 | 0.0860 | 0.0749 | 0.0678 | 0.0730 |
| #3 | 48 | 0.0990 | 0.0864 | 0.0786 | 0.0890 |
| #4 | 40 | 0.1120 | 0.0977 | 0.0896 | 0.0930 |
| #5 | 40 | 0.1250 | 0.1107 | 0.1014 | 0.1060 |
| #6 | 32 | 0.1380 | 0.1205 | 0.1113 | 0.1200 |
| #8 | 32 | 0.1640 | 0.1465 | 0.1375 | 0.1360 |
| #10 | 24 | 0.1900 | 0.1609 | 0.1511 | 0.1590 |
| 1/4″ | 20 | 0.2500 | 0.2175 | 0.2071 | 0.2010 |
| 5/16″ | 18 | 0.3125 | 0.2733 | 0.2622 | 0.2570 |
| 3/8″ | 16 | 0.3750 | 0.3277 | 0.3159 | 0.3120 |
| 7/16″ | 14 | 0.4375 | 0.3822 | 0.3697 | 0.3680 |
| 1/2″ | 13 | 0.5000 | 0.4339 | 0.4203 | 0.4210 |
| 9/16″ | 12 | 0.5625 | 0.4882 | 0.4739 | 0.4840 |
| 5/8″ | 11 | 0.6250 | 0.5421 | 0.5269 | 0.5310 |
| 3/4″ | 10 | 0.7500 | 0.6495 | 0.6335 | 0.6560 |
| 7/8″ | 9 | 0.8750 | 0.7565 | 0.7395 | 0.7660 |
| 1″ | 8 | 1.0000 | 0.8636 | 0.8466 | 0.9060 |
🔹 Note: The tap drill size is recommended for internal thread cutting.
UNC Standard Thread Dimensions in Metric (mm)
| Nominal Size | Major Diameter (mm) | Pitch Diameter (mm) | Minor Diameter (mm) | Tap Drill Size (mm) |
|---|---|---|---|---|
| #0 | 1.52 | 1.30 | 1.16 | 1.18 |
| #1 | 1.85 | 1.60 | 1.42 | 1.45 |
| #2 | 2.18 | 1.90 | 1.72 | 1.85 |
| #3 | 2.51 | 2.19 | 2.00 | 2.26 |
| #4 | 2.84 | 2.48 | 2.28 | 2.36 |
| #5 | 3.18 | 2.81 | 2.58 | 2.69 |
| #6 | 3.51 | 3.06 | 2.83 | 3.05 |
| #8 | 4.17 | 3.72 | 3.49 | 3.45 |
| #10 | 4.83 | 4.09 | 3.84 | 4.04 |
| 1/4″ | 6.35 | 5.52 | 5.26 | 5.11 |
| 5/16″ | 7.94 | 6.95 | 6.66 | 6.53 |
| 3/8″ | 9.53 | 8.32 | 8.02 | 7.92 |
| 7/16″ | 11.11 | 9.71 | 9.39 | 9.35 |
| 1/2″ | 12.70 | 11.02 | 10.67 | 10.69 |
| 9/16″ | 14.29 | 12.40 | 12.04 | 12.29 |
| 5/8″ | 15.88 | 13.77 | 13.39 | 13.49 |
| 3/4″ | 19.05 | 16.50 | 16.09 | 16.66 |
| 7/8″ | 22.23 | 19.22 | 18.79 | 19.46 |
| 1″ | 25.40 | 21.94 | 21.50 | 23.01 |
🔹 Note: The tap drill size is recommended for internal thread cutting.
Applications of UNC Threads
UNC threads are ideal for:
- Structural Fasteners (Bridges, buildings, and heavy-duty joints)
- Automotive Components (Chassis fasteners, engine mounts, suspension parts)
- Aerospace Applications (Where coarse threads are required in softer materials like aluminum.)
- General Manufacturing (Machinery, tools, industrial equipment.)
UNC threads are preferred over UNF (fine-pitch) threads in applications where:
✅ Quick assembly and disassembly are required
✅ Greater resistance to stripping is needed
✅ Threads will be exposed to vibration or high loads
Related Inch-Based Thread Standards
After reviewing UNC, you may also find the following standards useful:
- UNF – Unified National Fine (Higher strength in thinner materials.)
- UNEF – Unified National Extra Fine (Precision threads for specialized applications.)
- UNS – Unified National Special (Custom pitch threads for non-standard applications.)
- UNJ – Unified J-Form Threads (Modified UN threads with increased root radius for aerospace applications.)
- BSW – British Standard Whitworth (Traditional UK coarse threads.)
These links allow users to navigate easily between related standards.
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.