1/4-20 UNC Thread Quick Specifications
| Parameter | Specification | |
| Thread Standard | Unified Thread Standard (UTS) | |
| Thread Series | UNC | |
| Nominal Size | 1/4 | |
| Thread Designation | 1/4-20 UNC | |
| Threads Per Inch (TPI) | 20 | |
| Major Diameter | 0.250 in | 6.350 mm |
| Pitch | 0.050 in | 1.270 mm |
| Pitch Diameter | 0.2175 in | 5.525 mm |
| Minor Diameter | 0.1905 in | 4.839 mm |
| Thread Height | 0.0271 in | 0.688 mm |
| Addendum | 0.0162 in | 0.411 mm |
| Thread Angle | 60° | |
| Common Classes | 2A / 2B | |
| Typical Use | Precision electronic assemblies | |
Introduction
1/4-20 UNC is a thread defined within the Unified Thread Standard (UTS), a specification widely prevalent in North America and globally deployed across industrial and electronic hardware. Unified thread sizes follow an inch-based designation scheme.
This page serves as a full engineering reference covering 1/4-20 UNC dimensions and key specifications: thread designation, major diameter, pitch, threads per inch (TPI), minor diameter, and tolerance classes.
For engineers specifying threaded mounting hardware for feedthrough filters, EMI filters, RF shielding parts and precision mechanical assemblies, familiarity with the 1/4-20 UNC specification is critical to achieving good mechanical interoperability, robust installation, and consistent long-term operational performance.
Quick Answer: The 1/4-20 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.
1/4-20 UNC is a Unified National Coarse thread specification where:
- 1/4indicates the nominal screw size number (1/4 screw size)
- 20indicates 20 threads per inch (TPI)
- UNCindicates Unified National Coarse thread series
Per the Unified Thread Standard (UTS), the 1/4-20 UNC thread features a nominal major diameter of 0.250 inches (6.350 mm) with a standard 60-degree thread profile.
Boasting compact dimensions and sound mechanical strength, 1/4-20 UNC is a widely-specified miniature fastener for space-constrained electronic and mechanical designs.
Common applications include:
- Electronic enclosures
- PCB mounting hardware
- RF and microwave components
- EMI shielding assemblies
- Industrial control hardware
- High-precision opto-mechanical sub-systems
1/4-20 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: 1/4-20 UNC-2A
Internal Thread: 1/4-20 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: 1/4-20 UNC-3A
Internal Thread: 1/4-20 UNC-3B
Applications:
- Precision equipment
- High-vibration operating environments
- RF and microwave shielding assemblies
Characteristics:
- Reduced clearance
- Higher accuracy
- Higher manufacturing requirements
1/4-20 UNC vs Similar Thread Sizes
Engineers often need to determine whether another thread specification can replace 1/4-20 UNC.
1/4-20 UNC vs M6 × 1.0
This is one of the most common comparisons.
| Parameter | 1/4-20 UNC | M6 × 1.0 |
| Standard | Unified | Metric ISO |
| Major Diameter | 6.350 mm | 6.00 mm |
| Pitch | 1.270 mm | 1.00 mm |
| Pitch Diameter | 5.525 mm | 5.35 mm |
| Minor Diameter | 4.839 mm | 4.917 mm |
| Thread Height | 0.688 mm | 0.541 mm |
| Thread Angle | 60° | 60° |
Frequently Asked Questions (FAQ)
Q1. What does 1/4-20 UNC mean?
1/4-20 UNC is a Unified National Coarse thread designation. The number “1/4” represents the screw size number, “20” indicates 20 threads per inch (TPI), and “UNC” means Unified National Coarse thread series.
A 1/4-20 UNC thread has a nominal major diameter of approximately 0.250 inches (6.350 mm) and uses a 60-degree Unified thread profile.
Q2. Is 1/4-20 UNC a metric thread?
No. 1/4-20 UNC is an inch-based Unified thread specification, not a metric thread.
Although its diameter is similar to some metric screws such as M6, the thread pitch is different.
Q3. What tolerance class is commonly used for 1/4-20 UNC?
The most common tolerance class is:
- External thread: 1/4-20 UNC-2A
- Internal thread: 1/4-20 UNC-2B
Class 3A/3B is used when tighter precision is required.
Q4. Can LCA provide EMI filters with 1/4-20 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: Working on threaded-style EMI filter or feedthrough capacitor selection? Provide LCA engineering team with your thread specification, panel mounting thickness, electrical ratings (voltage / current), target capacitance value and environmental operating conditions for tailored technical guidance. View Thread Mount Feedthrough Filters
Related Engineering Resources
- #12-24 UNC Thread Dimensions
- #4-40 UNC Thread Dimensions
- 1/4-28 UNF Thread Dimensions
- #12-32 UNEF Thread Dimensions
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