1/4-28 UNF Thread Quick Specifications
| Parameter | Specification | |
| Thread Standard | Unified Thread Standard (UTS) | |
| Thread Series | UNF | |
| Nominal Size | 1/4 | |
| Thread Designation | 1/4-28 UNF | |
| Threads Per Inch (TPI) | 28 | |
| Major Diameter | 0.25 in | 6.35 mm |
| Pitch | 0.0357 in | 0.907 mm |
| Pitch Diameter | 0.2268 in | 5.761 mm |
| Minor Diameter | 0.2075 in | 5.271 mm |
| Thread Height | 0.0193 in | 0.49 mm |
| Addendum | 0.0116 in | 0.295 mm |
| Thread Angle | 60° | |
| Common Classes | 2A / 2B | |
| Typical Use | Precision electronic assemblies | |
Introduction
The 1/4-28 UNF thread is a Unified National Fine (UNF) thread, 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 1/4-28 UNF 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 1/4-28 UNF specification helps ensure proper mechanical compatibility, reliable installation, and long-term performance.
Quick Answer: The 1/4-28 UNF thread is a Unified National Fine (UNF) thread, which is primarily used in North America and widely adopted internationally in industrial and electronic applications.
1/4-28 UNF is a Unified National Fine thread specification where:
- 1/4indicates the nominal major diameter
- 28indicates 28 threads per inch (TPI)
- UNFindicates Unified National Fine thread series
1/4-28 UNF 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: 1/4-28 UNF-2A
Internal Thread: 1/4-28 UNF-2B
Characteristics:
- Easiest assembly
- Best interchangeability
- Most industrial applications
Most commercial fasteners use Class 2.
Class 3A / 3B Threads
Class 3 provides a tighter fit.
External Thread: 1/4-28 UNF-3A
Internal Thread: 1/4-28 UNF-3B
Applications:
- Precision equipment
- Aerospace systems
- High-performance assemblies
Characteristics:
- Reduced clearance
- Higher positional accuracy
- Tighter manufacturing control
1/4-28 UNF vs Similar Thread Sizes
Engineers often need to determine whether another thread specification can replace 1/4-28 UNF.
1/4-28 UNF vs M6 × 1
| Parameter | 4-40 UNC | M3 × 0.5 |
| Standard | Unified | Metric ISO |
| Major Diameter | 6.35 mm | 6.00 mm |
| Pitch | 0.907 mm | 1 mm |
| Pitch Diameter | 5.761 mm | 5.35 mm |
| Minor Diameter | 5.271 mm | 4.917 mm |
| Thread Height | 0.49 mm | 0.541 mm |
| Thread Angle | 60° | 60° |
Frequently Asked Questions (FAQ)
Q1. What is the pitch of a 1/4-28 UNF thread?
28 TPI, equivalent to 0.907 mm pitch.
Q2. Is 1/4-28 UNF a metric thread?
No. 1/4-28 UNF 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 1/4-28 UNF?
The most common tolerance class is:
- External thread: 1/4-28 UNF-2A
- Internal thread: 1/4-28 UNF-2B
Class 3A/3B is used when tighter precision is required.
Q4. Is 1/4-28 UNF suitable for EMI filters?
Yes. It is commonly used in precision panel-mounted EMI and RF assemblies.
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
Thread Knowledge
- #4-40 UNC Thread Dimensions
- #10-32UNF Thread Dimensions
- 5/8-18 UNF Thread Dimensions
- #12-32 UNEF Thread Dimensions
- 9/16-24 UNEF Thread Dimensions
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