3-1/4-4 UNC Thread Specifications
Complete dimensions for 3-1/4-4 UNC thread per ASME B1.1. TPI: 4, Pitch: 0.25" (6.35 mm).
Thread Dimensions
| Designation | 3-1/4-4 UNC |
| Threads Per Inch (TPI) | 4 |
| Pitch | 0.25" (6.35 mm) |
| Major Diameter | 3.25" (82.55 mm) |
| Pitch Diameter | 3.0876" (78.426 mm) |
| Minor Diameter (External) | 2.9433" (74.759 mm) |
| Minor Diameter (Internal) | 2.9794" (75.676 mm) |
| Thread Depth | 0.13532" (3.437 mm) |
| Tensile Stress Area | 7.05637 in² (4552.5 mm²) |
Tap Drill Size
Recommended tap drill: 1/1" (1.0" / 25.4 mm)
Calculated minor diameter (internal): 2.9794" (75.68 mm)
Proof Loads by SAE Grade
| Grade | Proof Stress (psi) | Tensile Strength (psi) | Proof Load (lbf) | Tensile Load (lbf) |
|---|---|---|---|---|
| Grade 2 | 55,000 | 74,000 | 388,100 | 522,171 |
| Grade 5 | 85,000 | 120,000 | 599,791 | 846,764 |
| Grade 8 | 120,000 | 150,000 | 846,764 | 1,058,456 |
Common Applications
- Industrial and structural applications
- Coarse thread applications