3 3/4-4 UNC Thread Quick Specifications
| Parameter | Specification | |
| Thread Standard | Unified Thread Standard (UTS) | |
| Thread Series | UNC | |
| Nominal Size | 3 3/4 | |
| Thread Designation | 3 3/4-4 UNC | |
| Threads Per Inch (TPI) | 4 | |
| Major Diameter | 3.7500 in | 95.250 mm |
| Pitch | 0.2500 in | 6.3500 mm |
| Pitch Diameter | 3.5876 in | 91.125 mm |
| Minor Diameter | 3.4523 in | 87.688 mm |
| Thread Height | 0.1353 in | 3.4370 mm |
| Addendum | 0.0812 in | 2.0620 mm |
| Thread Angle | 60° | |
| Common Classes | 2A / 2B | |
| Typical Use | Precision electronic assemblies | |
Overview
Belonging to the Unified Thread Standard (UTS), the 3 3/4-4 UNC coarse thread is a widely adopted inch-series fastener standard originating from North America and deployed globally across industrial, aerospace and RF electronic fields. Its standardized geometric profile guarantees cross-manufacturer part interchangeability and consistent assembly results for worldwide supply chains.
This technical reference page summarizes key dimensional data, tolerance grades and comparison information for 3 3/4-4 UNC. Engineers can reference major diameter, thread pitch, minor diameter and tolerance limits to design panel cutouts, threaded feedthrough filter installations and mechanical joint structures.
For design specialists developing EMI shielding housings, filter feedthrough assemblies and precision electromechanical hardware, mastering the geometric properties and tolerance limits of 3 3/4-4 UNC is essential to maintain structural integrity, preserve shielding continuity and sustain stable system performance under harsh working environments.
Quick Answer:
The 3 3/4-4 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.
3 3/4-4 UNC is a Unified National Coarse thread specification where:
- 33/4 indicates the nominal screw size number (3 3/4 screw size)
- 4indicates 4 threads per inch (TPI)
- UNCindicates Unified National Coarse thread series
This thread conforms to the Unified Thread Standard (UTS)-defined 60-degree unified thread form, with a nominal major diameter of 3.750 inches (95.250 mm).
It provides robust tensile and shear strength while maintaining a compact outer profile, making it well-suited for structural fastening in space-constrained panel mounting applications. Consequently, it is frequently specified in high-performance EMI shielding enclosures and precision mechanical assemblies.
Common applications include:
- Shielding enclosures for power conversion equipment
- Precision multi-part mechanical subassemblies
- Threaded fastening hardware for board-level mounting
- RF and microwave waveguide & shielding components
- EMI containment structural assemblies
- Metrology and measurement test instruments
3 3/4-4 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: 3 3/4-4 UNC-2A
Internal Thread: 3 3/4-4 UNC-2B
Characteristics:
- Balanced fit performance
- Good manufacturability
- Cost-effective for large-batch manufacturing
- Optimized clearance for easy assembly
- 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: 3 3/4-4 UNC-3A
Internal Thread: 3 3/4-4 UNC-3B
Applications:
- Ultra-precision industrial machine assemblies
- Aerospace and airborne electronic systems
- Mission-critical mechanical structures requiring high stability
Characteristics:
- Reduced clearance
- Higher accuracy requirements
- Stricter manufacturing and inspection standards
- Designed for high-reliability, mission-critical hardware
3 3/4-4 UNC vs Similar Thread Sizes
Engineers often need to determine whether another thread specification can replace 3 3/4-4 UNC.
3 3/4-4 UNC vs M95 × 6.0
This is one of the most common comparisons.
| Parameter | 3 3/4-4 UNC | M95 × 6.0 |
| Standard | Unified | Metric ISO |
| Major Diameter | 95.250 mm | 95.0 mm |
| Pitch | 6.3500 mm | 6.00 mm |
| Pitch Diameter | 91.125 mm | 91.103 mm |
| Minor Diameter | 87.688 mm | 88.505 mm |
| Thread Height | 3.4370 mm | 3.2480 mm |
| Thread Angle | 60° | 60° |
Frequently Asked Questions (FAQ)
Q1. What does the designation 3 3/4-4 UNC stand for?
3 3/4-4 UNC is a Unified National Coarse thread designation. The number “3 3/4” represents the nominal screw size, “4” indicates 4 threads per inch (TPI), and “UNC” means Unified National Coarse thread series.
A 3 3/4-4 UNC thread has a nominal major diameter of approximately 3.750 inches (95.250 mm) and uses a 60-degree Unified thread profile.
Q2. Is 3 3/4-4 UNC categorized as a metric thread?
No. 3 3/4-4 UNC is an inch-based Unified thread specification, not a metric thread.
Although its diameter is similar to some metric screws such as M95, the thread pitch is different.
Q3. What tolerance class is commonly used for 3 3/4-4 UNC?
The most common tolerance class is:
- External thread: 3 3/4-4 UNC-2A
- Internal thread: 3 3/4-4 UNC-2B
Class 3A/3B is used when tighter precision is required.
Q4. Is 3 3/4-4 UNC interchangeable with other similar-sized threads?
Interchange is not allowed. Despite the similar outer diameter with M95 metric thread or other UN fine threads, the unique TPI and pitch of 3 3/4-4 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: Looking to specify threaded feedthrough capacitors or EMI filters? Send your thread specification, panel thickness rating, voltage and current requirements, target capacitance value and environmental operating conditions to our LCA engineering team to receive customized technical recommendations and product solutions. View Thread Mount Feedthrough Filters
Related Engineering Resources
- #8-32 UNC Thread Dimensions
- 3 1/4-4 UNC Thread Dimensions
- #4-40 UNC Thread Dimensions
- 8-36UNF Thread Dimensions
- #12-32 UNEF Thread Dimensions
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