#4-40 UNC Thread Quick Specifications
| Parameter | Specification | |
| Thread Standard | Unified Thread Standard (UTS) | |
| Thread Series | UNC | |
| Nominal Size | #4 | |
| Thread Designation | #4-40 UNC | |
| Threads Per Inch (TPI) | 40 | |
| Major Diameter | 0.1120 in | 2.845 mm |
| Pitch | 0.025 in | 0.635 mm |
| Pitch Diameter | 0.0958 in | 2.433 mm |
| Minor Diameter | 0.0822 in | 2.088 mm |
| Thread Height | 0.0135 in | 0.343 mm |
| Addendum | 0.0081 in | 0.206 mm |
| Thread Angle | 60° | |
| Common Classes | 2A / 2B | |
| Typical Use | Precision electronic assemblies | |
Introduction
The #4-40 UNC thread belongs to the Unified Thread Standard (UTS), which is primarily used in North America and widely adopted internationally in industrial and electronic applications. Unified threads are identified using an inch-based numbering system.
This page provides a complete engineering reference for #4-40 UNC thread dimensions and specifications, including thread designation, major diameter, pitch, threads per inch (TPI), minor diameter, tolerance classes.
For engineers selecting threaded mounting solutions for electronic components such as feedthrough filters, EMI filters, RF shielding components, and precision assemblies, understanding the #4-40 UNC specification helps ensure proper mechanical compatibility, reliable installation, and long-term performance.
Quick Answer: The #4-40 UNC thread belongs to the Unified Thread Standard (UTS), which is primarily used in North America and widely adopted internationally in industrial and electronic applications.
#4-40 UNC is a Unified National Coarse thread specification where:
- #4indicates the nominal screw size number (#4 screw size)
- 40indicates 40 threads per inch (TPI)
- UNCindicates Unified National Coarse thread series
A #4-40 UNC thread has a nominal major diameter of approximately 0.112 inches (2.84 mm) and uses a 60-degree thread profile according to the Unified Thread Standard (UTS).
Because of its compact size and reliable mechanical strength, 4-40 UNC threads are frequently used in applications requiring miniature fastening solutions, especially where space is limited.
Common applications include:
- Electronic enclosures
- PCB mounting hardware
- RF and microwave components
- EMI shielding assemblies
- Instrumentation equipment
- Aerospace electronics
- Military electronic systems
- Precision mechanical assemblies
#4-40 UNC Thread Tolerance Classes
Thread tolerance determines how tightly two mating components fit together.
The Unified Thread Standard defines several tolerance classes.
Class 2A / 2B Threads
Class 2 is the most commonly used commercial tolerance.
External Thread: 4-40 UNC-2A
Internal Thread: 4-40 UNC-2B
Characteristics:
- Balanced fit
- Good manufacturability
- Suitable for general industrial applications
Most commercial fasteners use Class 2.
Class 3A / 3B Threads
Class 3 provides a tighter fit.
External Thread: 4-40 UNC-3A
Internal Thread: 4-40 UNC-3B
Applications:
- Precision equipment
- Aerospace systems
- High-performance assemblies
Characteristics:
- Reduced clearance
- Higher accuracy
- Higher manufacturing requirements
4-40 UNC vs Similar Thread Sizes
Engineers often need to determine whether another thread specification can replace 4-40 UNC.
4-40 UNC vs M3 × 0.5
This is one of the most common comparisons.
| Parameter | 4-40 UNC | M3 × 0.5 |
| Standard | Unified | Metric ISO |
| Major Diameter | 2.845 mm | 3.00 mm |
| Pitch | 0.635 mm | 0.5 mm |
| Pitch Diameter | 2.433 mm | 2.675 mm |
| Minor Diameter | 2.088 mm | 2.459 mm |
| Thread Height | 0.343 mm | 0.271 mm |
| Thread Angle | 60° | 60° |
Frequently Asked Questions (FAQ)
Q1. What does 4-40 UNC mean?
4-40 UNC is a Unified National Coarse thread designation. The number “4” represents the screw size number, “40” indicates 40 threads per inch (TPI), and “UNC” means Unified National Coarse thread series.
A 4-40 UNC thread has a nominal major diameter of approximately 0.112 inches (2.84 mm) and uses a 60-degree Unified thread profile.
Q2. Is 4-40 UNC a metric thread?
No. 4-40 UNC is an inch-based Unified thread specification, not a metric thread.
Although its diameter is similar to some metric screws such as M3, the thread pitch is different.
Q3. What tolerance class is commonly used for 4-40 UNC?
The most common tolerance class is:
- External thread: 4-40 UNC-2A
- Internal thread: 4-40 UNC-2B
Class 3A/3B is used when tighter precision is required.
Q4. Can LCA provide EMI filters with 4-40 UNC threads?
Yes. LCA supports customized EMI feedthrough filter designs with various thread configurations, including miniature inch and metric thread options.
Customization may include:
- Thread size
- Housing dimensions
- Electrical parameters
- Sealing structure
- Plating options
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
- #8-32 UNC Thread Dimensions
- 3/8-16 UNC Thread Dimensions
- 1/4-28 UNF Thread Dimensions
- #12-32 UNEF Thread Dimensions
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