5/8-18 UNF Thread Quick Specifications
| Parameter | Specification | |
| Thread Standard | Unified Thread Standard (UTS) | |
| Thread Series | UNF | |
| Nominal Size | 5/8 | |
| Thread Designation | 5/8-18 UNF | |
| Threads Per Inch (TPI) | 18 | |
| Major Diameter | 0.6250 in | 15.875 mm |
| Pitch | 0.0556 in | 1.4120 mm |
| Pitch Diameter | 0.5889 in | 14.958 mm |
| Minor Diameter | 0.5588 in | 14.194 mm |
| Thread Height | 0.0301 in | 0.7650 mm |
| Addendum | 0.0180 in | 0.4750 mm |
| Thread Angle | 60° | |
| Common Classes | 2A / 2B | |
| Typical Use | Precision electronic assemblies | |
Introduction
The 5/8-18 UNF thread is a member of the Unified National Fine (UNF) threadseries. Primarily originating from North-American standards, it has gained broad international acceptance across industrial and electronic applications. Unified threads follow an inch-based numbering convention.
The 5/8-18 UNF thread features a nominal major diameter of 0.625 inches (15.875 mm) and a pitch of 0.0556 inches (1.412 mm). The most widely-used tolerance classes are 2A for external threads and 2B for internal threads.
This page delivers a full engineering reference for 5/8-18 UNF thread dimensions and specifications, covering thread designation, major diameter, pitch, threads per inch (TPI), minor diameter, and tolerance classes.
For engineers specifying threaded mounting hardware for electronic components—including feedthrough filters, EMI filters, RF shielding parts, and precision mechanical assemblies—familiarity with the 5/8-18 UNF specification ensures proper mechanical mating, dependable installation, and stable long-term component performance.
Quick Answer
The 5/8-18 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.
5/8-18 UNF is a Unified National Fine thread specification defined as:
- 5/8indicates the nominal major diameter
- 18indicates 18 threads per inch (TPI)
- UNFdenotes the Unified National Fine thread series—it features a finer pitch (more threads per inch) compared with UNC, delivering improved strength and vibration resistance, yet it is more susceptible to cross-threading.
5/8-18 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. Close to 90% of all commercial and industrial fasteners produced in North America use this class of thread fit.
External Thread: 5/8-18 UNF-2A
Internal Thread: 5/8-18 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: 5/8-18 UNF-3A
Internal Thread: 5/8-18 UNF-3B
Applications:
- Precision equipment
- Aerospace systems
- High-performance assemblies
Characteristics:
- Reduced clearance
- Higher positional accuracy
- Tighter manufacturing control
5/8-18 UNF vs Similar Thread Sizes
Engineers often need to determine whether another thread specification can replace 5/8-18 UNF.
5/8-18 UNF vs M16 × 1.5
| Parameter | 5/8-18 UNF | M16 × 1.5 |
| Standard | Unified | Metric ISO |
| Major Diameter | 15.875 mm | 16.00 mm |
| Pitch | 1.4120 mm | 1.500 mm |
| Pitch Diameter | 14.958 mm | 15.026 mm |
| Minor Diameter | 14.194 mm | 14.376 mm |
| Thread Height | 0.7650 mm | 0.812 mm |
| Thread Angle | 60° | 60° |
Frequently Asked Questions (FAQ)
Q1. What is the pitch of a 5/8-18 UNF thread?
18 TPI, equivalent to 1.411 mm pitch.
Q2. Is 5/8-18 UNF a metric thread?
No. 5/8-18 UNF is an inch-based Unified thread specification, not a metric thread.
While its nominal diameter is close to M16, their thread pitches are different.
Q3. What tolerance class is commonly used for 5/8-18 UNF?
The most common tolerance class is:
- External thread: 5/8-18 UNF-2A
- Internal thread: 5/8-18 UNF-2B
Class 3A/3B is used when tighter precision is required.
Q4. Is 5/8-18 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
- 5/16-18 UNC Thread Dimensions
- #8-36UNF Thread Dimensions
- 1/4-28UNF Thread Dimensions
- 7/8-20 UNEF Thread Dimensions
- 9/16-24 UNEF Thread Dimensions
- M3 Thread Dimensions
- M18 Thread Dimensions
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