Introduction
Quick Answer: UNC (Unified National Coarse) is the most commonly used thread series within the Unified Thread Standard (UTS).
UNC (Unified National Coarse) is the most commonly used thread series within the Unified Thread Standard (UTS). Designed with a coarse thread pitch, UNC threads provide an excellent balance of strength, ease of assembly, and durability, making them the preferred choice for general-purpose fastening in a wide range of industrial applications.
From mechanical equipment and electronic enclosures to threaded feedthrough filters and automation systems, UNC threads are trusted wherever reliable fastening and broad compatibility are required.
Quick Definition
UNC (Unified National Coarse) is the coarse-pitch thread series defined by the Unified Thread Standard (UTS). Compared with fine-thread series such as UNF, UNC threads have fewer threads per inch (TPI), resulting in deeper thread profiles, improved resistance to thread damage, and easier assembly.
What Are UNC Threads?
UNC threads are standardized inch-based screw threads designed primarily for general engineering applications.
Their relatively coarse pitch allows fast installation, reduced sensitivity to contamination, and better performance in softer materials such as aluminum, brass, and plastics.
Because of these characteristics, UNC has become the default thread series for many mechanical assemblies throughout North America.
Common examples include:
Each size follows standardized dimensions defined by ASME B1.1, ensuring compatibility between components produced by different manufacturers.
History & Development
The Unified National Coarse thread series was established as part of the Unified Thread Standard to provide a common inch-based fastening system across the United States, Canada, and the United Kingdom.
UNC quickly became the preferred general-purpose thread because its coarse pitch offered reliable assembly, good manufacturing tolerance, and excellent interchangeability.
Today, UNC remains one of the most widely used thread series in industrial manufacturing and maintenance.
Key Characteristics
| Characteristic | Specification |
| Standard | ASME B1.1 |
| Measurement System | Inch |
| Thread Angle | 60° |
| Thread Pitch | Coarse |
| Pitch Unit | Threads Per Inch (TPI) |
| Common Thread Classes | 2A, 2B, 3A, 3B |
| Primary Use | General-purpose fastening |
Understanding UNC Thread Designations
A standard UNC designation consists of four parts:
D – P S – C
Nominal Diameter – Threads Per Inch Thread Series – Thread Class
Nominal Diameter:
- For diameters of 1/4″ and above, use fractional inches. For example 1/4″, 5/16″.
- For diameters smaller than 1/4″, they are indicated by numbers ranging from #0 to #12, with each “number designation” corresponding to a specific diameter, as shown in the table below.
Diameter Number | #0 | #1 | #2 | #3 | #4 | #5 | #6 | #8 | #10 | #12 |
| Diameter (Inch) | 0.06 | 0.073 | 0.086 | 0.099 | 0.112 | 0.125 | 0.138 | 0.164 | 0.19 | 0.216 |
| Diameter (mm) | 1.524 | 1.8542 | 2.1844 | 2.5146 | 2.8448 | 3.175 | 3.5052 | 4.1656 | 4.8260 | 5.4864 |
Pitch:
The pitch is given in Threads Per Inch (TPI).
Thread Series：
Thread Class：
This class is defined by two characters. For example 1A, 2A, 3A, 1B, 2B or 3B.
- First character is a number from 1 to 3: 1 = Loose fit, 2 = Medium fit, and 3 = Tight fit.
- Second character is a letter: A =External thread, and B = Internal thread.
Example:
1/4-20 UNC-2A
| Element | Description |
| 1/4 | Nominal Diameter |
| 20 | Threads Per Inch |
| UNC | Unified National Coarse |
| 2A | External Thread Class |
This naming convention enables engineers to identify the thread dimensions and tolerance requirements at a glance.
Advantages of UNC Threads
UNC threads are widely adopted because they offer several practical engineering advantages:
- Fast and easy assembly.
- Greater resistance to cross-threading.
- Better performance in softer materials such as aluminum and brass.
- Improved resistance to dirt and minor surface damage.
- Broad availability of standard fasteners and tooling.
- Lower manufacturing cost compared with finer thread series.
These characteristics make UNC the preferred choice for many general industrial applications.
Limitations
Although versatile, UNC threads are not always the optimal solution.
Consider alternatives when:
- Higher clamping force is required.
- Fine adjustment is important.
- Space is limited.
- Thin-wall components are used.
- High-vibration environments require specialized fastening strategies.
In these cases, UNF or UNEF threads may provide better performance.
Typical Applications
UNC threads are commonly used across a wide range of industries.
Typical applications include:
- Industrial machinery
- Automation equipment
- Electronic enclosures
- Telecommunications hardware
- Test and measurement instruments
- Military equipment
- Agricultural machinery
- Construction equipment
- Threaded feedthrough filters
- EMI shielding products
For manufacturers such as LCA, UNC threads are commonly specified on thread-mounted feedthrough filters intended for North American equipment and panel installations.
UNC vs UNF
| Feature | UNC | UNF |
| Thread Pitch | Coarse | Fine |
| Threads Per Inch | Fewer | More |
| Assembly Speed | Faster | Slower |
| Thread Strength | Better in soft materials | Higher preload |
| Vibration Resistance | Good | Better |
| Typical Applications | General machinery | Aerospace, automotive, precision equipment |
UNC vs ISO Metric Threads
| UNC | ISO Metric |
| Inch dimensions | Metric dimensions |
| Pitch defined by TPI | Pitch defined in millimeters |
| ASME B1.1 | ISO 68 / ISO 261 / ISO 965 |
| North American standard | International standard |
Although both use a 60° thread angle, they are not interchangeable due to differences in dimensions and pitch.
Engineering Considerations
When selecting UNC threads, engineers should evaluate:
- Material strength
- Required clamping force
- Installation frequency
- Vibration conditions
- Thread engagement length
- Manufacturing capabilities
- Compatibility with existing equipment
Choosing the appropriate thread class (2A/2B or 3A/3B) is equally important to ensure proper fit and performance.
UNC Series Basic Dimensions chart
Click Thread Link to check more data for each thread.
| Thread | Threads per inch (TPI) | Basic Major Diameter | Basic Minor Diameter | Thread Height | |||
| (in) | (mm) | (in) | (mm) | (in) | (mm) | ||
| #1 – 64 UNC | 64 | 0.073 | 1.854 | 0.0544 | 1.382 | 0.0085 | 0.216 |
| #2 – 56 UNC | 56 | 0.086 | 2.184 | 0.0647 | 1.643 | 0.0097 | 0.246 |
| #3 – 48 UNC | 48 | 0.099 | 2.515 | 0.0742 | 1.885 | 0.0113 | 0.287 |
| #4 – 40 UNC | 40 | 0.112 | 2.845 | 0.0822 | 2.088 | 0.0135 | 0.343 |
| #5 – 40 UNC | 40 | 0.125 | 3.175 | 0.0952 | 2.418 | 0.0135 | 0.343 |
| #6 – 32 UNC | 32 | 0.138 | 3.505 | 0.1008 | 2.56 | 0.0169 | 0.429 |
| #8 – 32 UNC | 32 | 0.164 | 4.166 | 0.1268 | 3.221 | 0.0169 | 0.429 |
| #10 – 24 UNC | 24 | 0.19 | 4.826 | 0.1404 | 3.566 | 0.0226 | 0.574 |
| #12 – 24 UNC | 24 | 0.216 | 5.486 | 0.1664 | 4.227 | 0.0226 | 0.574 |
| 1/4 – 20 UNC | 20 | 0.25 | 6.35 | 0.1905 | 4.839 | 0.0271 | 0.688 |
| 5/16 – 18 UNC | 18 | 0.313 | 7.938 | 0.2463 | 6.256 | 0.0301 | 0.765 |
| 3/8 – 16 UNC | 16 | 0.375 | 9.525 | 0.3006 | 7.635 | 0.0338 | 0.859 |
| 7/16 – 14 UNC | 14 | 0.438 | 11.112 | 0.3524 | 8.951 | 0.0387 | 0.983 |
| 1/2 – 13 UNC | 13 | 0.5 | 12.7 | 0.4084 | 10.373 | 0.0416 | 1.057 |
| 9/16 – 12 UNC | 12 | 0.563 | 14.288 | 0.4633 | 11.768 | 0.0451 | 1.146 |
| 5/8 – 11 UNC | 11 | 0.625 | 15.875 | 0.5167 | 13.124 | 0.0492 | 1.25 |
| 3/4 – 10 UNC | 10 | 0.75 | 19.05 | 0.6309 | 16.025 | 0.0541 | 1.374 |
| 7/8 – 9 UNC | 9 | 0.875 | 22.225 | 0.7427 | 18.865 | 0.0601 | 1.527 |
| 1 – 8 UNC | 8 | 1 | 25.4 | 0.8512 | 21.62 | 0.0677 | 1.72 |
| 1 1/8 – 7 UNC | 7 | 1.125 | 28.575 | 0.9549 | 24.254 | 0.0773 | 1.963 |
| 1 1/4 – 7 UNC | 7 | 1.25 | 31.75 | 1.0799 | 27.429 | 0.0773 | 1.963 |
| 1 3/8 – 6 UNC | 6 | 1.375 | 34.925 | 1.1766 | 29.886 | 0.0902 | 2.291 |
| 1 1/2 – 6 UNC | 6 | 1.5 | 38.1 | 1.3015 | 33.058 | 0.0902 | 2.291 |
| 1 3/4 – 5 UNC | 5 | 1.75 | 44.45 | 1.5118 | 38.4 | 0.1083 | 2.751 |
| 2 – 4.5 UNC | 4.5 | 2 | 50.8 | 1.7354 | 44.079 | 0.1203 | 3.056 |
| 2 1/4 – 4.5 UNC | 4.5 | 2.25 | 57.15 | 1.9523 | 49.588 | 0.1203 | 3.056 |
| 2 1/2 – 4 UNC | 4 | 2.5 | 63.5 | 2.2023 | 55.938 | 0.1353 | 3.437 |
| 2 3/4 – 4 UNC | 4 | 2.75 | 69.85 | 2.4523 | 62.288 | 0.1353 | 3.437 |
| 3 – 4 UNC | 4 | 3 | 76.2 | 2.7023 | 68.638 | 0.1353 | 3.437 |
| 3 1/4 – 4 UNC | 4 | 3.25 | 82.55 | 2.9523 | 74.988 | 0.1353 | 3.437 |
| 3 1/2 – 4 UNC | 4 | 3.5 | 88.9 | 3.2023 | 81.338 | 0.1353 | 3.437 |
| 3 3/4 – 4 UNC | 4 | 3.75 | 95.25 | 3.4523 | 87.688 | 0.1353 | 3.437 |
| 4 – 4 UNC | 4 | 4 | 101.6 | 3.7023 | 94.038 | 0.1353 | 3.437 |
Frequently Asked Questions
- What does UNC stand for?
Unified National Coarse.
- Why are UNC threads called “coarse”?
Because they have fewer threads per inch than UNF threads, resulting in a larger thread pitch and deeper thread profile.
- Are UNC threads stronger than UNF threads?
Not necessarily. UNC threads generally perform better in softer materials, while UNF threads often provide higher preload and improved vibration resistance in precision applications.
- Are UNC and Metric threads compatible?
No. Although both systems use a 60° thread angle, they differ in dimensions, pitch, and standards.
- Which UNC thread size is most common?
Sizes such as 4-40 UNC, 6-32 UNC, 8-32 UNC, and 1/4-20 UNC are among the most widely used.
Engineering Support CTA: Selecting a threaded EMI filter or feedthrough capacitor? Share the thread standard and size, panel thickness, voltage/current, capacitance, and environmental requirements with LCA engineers. View Thread Mount Feedthrough Filters
Related Engineering Resources
Related LCA Products
Continue Exploring LCA Thread Engineering Center
Whether you are selecting a standard fastener, designing a threaded EMI filter, or identifying a thread specification, understanding UNC threads is the first step toward choosing the correct fastening solution.
Browse our Thread Standards, Thread Charts, Thread Specifications, and Engineering Guides to access detailed engineering data, application recommendations, and dimensional references.
