7/8-9 UNC Thread Quick Specifications
| Parameter | Specification | |
| Thread Standard | Unified Thread Standard (UTS) | |
| Thread Series | UNC | |
| Nominal Size | 7/8 | |
| Thread Designation | 7/8-9 UNC | |
| Threads Per Inch (TPI) | 9 | |
| Major Diameter | 0.8750 in | 22.225 mm |
| Pitch | 0.1111 in | 2.8220 mm |
| Pitch Diameter | 0.8028 in | 20.391 mm |
| Minor Diameter | 0.7427 in | 18.865 mm |
| Thread Height | 0.0601 in | 1.5270 mm |
| Addendum | 0.0361 in | 0.9170 mm |
| Thread Angle | 60° | |
| Common Classes | 2A / 2B | |
| Typical Use | Precision electronic assemblies | |
Introduction
As a fundamental specification within the Unified Thread Standard (UTS) — the predominant inch-based thread system in North America — the 7/8-9 UNC thread plays a critical role in numerous industrial and electronic applications. This thread series is defined by a standardized dimensional framework that ensures interchangeability and assembly consistency across global supply chains.
This page provides a systematic engineering reference for the 7/8-9 UNC thread, detailing its major diameter, pitch, threads per inch (TPI), minor diameter, and tolerance classes. This thread size is commonly employed in power electronics, filter assemblies, RF shielding structures, and heavy-duty mechanical connections.
For engineers and technical professionals engaged in the design and integration of threaded mounting solutions for filters, shielding enclosures, and precision electromechanical systems, a comprehensive understanding of the 7/8-9 UNC thread’s geometric characteristics and tolerance framework is essential to ensure mechanical robustness, achieve reliable electrical contact, and guarantee long-term system performance under demanding operating conditions.
Quick Answer: The 7/8-9 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.
7/8-9 UNC is a Unified National Coarse thread specification where:
- 7/8indicates the nominal screw size number (7/8 screw size)
- 9indicates 9 threads per inch (TPI)
- UNCindicates Unified National Coarse thread series
Defined by the Unified Thread Standard (UTS), the 7/8-9 UNC adopts the standard 60-degree unified thread profile, with a nominal major diameter measuring 0.875 inches (22.225 mm).
Boasting solid mechanical strength while keeping a reasonable overall dimension, this thread size is well-suited for mounting tasks where available installation space is limited. It has gained extensive adoption within high-reliability electronics and mechanical engineering sectors.
Common applications include:
- Equipment housing structures
- High-precision mechanical subassemblies
- PCB-related fastening components
- RF & microwave hardware assemblies
- EMI-shielded structural sub-assemblies
- Test and measurement instrumentation
- Avionics and aerospace-grade electronics
7/8-9 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: 7/8-9 UNC-2A
Internal Thread: 7/8-9 UNC-2B
Characteristics:
- Balanced fit performance
- Good manufacturability
- Low mass-production costs
- Suitable for general industrial applications and standard fasteners
Most commercial fasteners use Class 2.
Class 3A / 3B Threads
Class 3 provides a tighter fit.
External Thread: 7/8-9 UNC-3A
Internal Thread: 7/8-9 UNC-3B
Applications:
- High-precision industrial machinery
- Aerospace and airborne electronic systems
- Mission-critical mechanical structures requiring high stability
Characteristics:
- Reduced clearance
- Higher accuracy requirements
- Stricter manufacturing and inspection standards
- Suitable for high-reliability and high-performance applications
7/8-9 UNC vs Similar Thread Sizes
Engineers often need to determine whether another thread specification can replace 7/8-9 UNC.
7/8-9 UNC vs M22 × 2.5
This is one of the most common comparisons.
| Parameter | 7/8-9 UNC | M22 × 2.5 |
| Standard | Unified | Metric ISO |
| Major Diameter | 22.225 mm | 22.0 mm |
| Pitch | 2.8220 mm | 2.50 mm |
| Pitch Diameter | 20.391 mm | 20.376 mm |
| Minor Diameter | 18.865 mm | 19.294 mm |
| Thread Height | 1.5270 mm | 1.353 mm |
| Thread Angle | 60° | 60° |
Frequently Asked Questions (FAQ)
Q1. What does 7/8-9 UNC mean?
7/8-9 UNC is a Unified National Coarse thread designation. The number “7/8” represents the screw size number, “9” indicates 9 threads per inch (TPI), and “UNC” means Unified National Coarse thread series.
A 7/8-9 UNC thread has a nominal major diameter of approximately 0.875 inches (22.225 mm) and uses a 60-degree Unified thread profile.
Q2. Is 7/8-9 UNC a metric thread?
No. 7/8-9 UNC is an inch-based Unified thread specification, not a metric thread.
Although its diameter is similar to some metric screws such as M22, the thread pitch is different.
Q3. What tolerance class is commonly used for 7/8-9 UNC?
The most common tolerance class is:
- External thread: 7/8-9 UNC-2A
- Internal thread: 7/8-9 UNC-2B
Class 3A/3B is used when tighter precision is required.
Q4. What application scenarios are 7/8-9 UNC threaded EMI filters suitable for?
Featuring reliable mechanical strength and universal mounting compatibility, 7/8-9 UNC threaded EMI filters are commonly used for aerospace electronics, precision mechanical assemblies, industrial instrumentation, and RF/microwave shielding systems, ensuring stable EMI suppression and long-term assembly reliability in high-reliability electronic equipment.
Q5. Is 7/8-9 UNC interchangeable with other similar-sized threads?
Interchange is not allowed. Despite the similar outer diameter with M22 metric thread or other UN fine threads, the unique TPI and pitch of 7/8-9 UNC result in mismatched meshing teeth. Mismatched thread assembly will cause thread damage, poor sealing, loose installation, and even failure of EMI filter shielding performance.
Engineering Support CTA: Selecting a threaded EMI filter or feedthrough capacitor? Share the thread standard and size, panel thickness, voltage/current ratings, target capacitance value, and environmental operating requirements with LCA engineers to obtain professional tailored technical support and product solutions. View Thread Mount Feedthrough Filters
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