1/4-20 UNC Thread Specifications
Complete dimensions for 1/4-20 UNC thread per ASME B1.1. TPI: 20, Pitch: 0.05" (1.27 mm).
Thread Dimensions
| Designation | 1/4-20 UNC |
| Threads Per Inch (TPI) | 20 |
| Pitch | 0.05" (1.27 mm) |
| Major Diameter | 0.25" (6.35 mm) |
| Pitch Diameter | 0.2175" (5.525 mm) |
| Minor Diameter (External) | 0.1887" (4.792 mm) |
| Minor Diameter (Internal) | 0.1959" (4.975 mm) |
| Thread Depth | 0.02706" (0.687 mm) |
| Tensile Stress Area | 0.03125 in² (20.2 mm²) |
Tap Drill Size
Recommended tap drill: #9 (0.196" / 4.98 mm)
Calculated minor diameter (internal): 0.1959" (4.97 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 | 1,719 | 2,312 |
| Grade 5 | 85,000 | 120,000 | 2,656 | 3,750 |
| Grade 8 | 120,000 | 150,000 | 3,750 | 4,688 |
Same Size, Different Series
Common Applications
- General purpose fastening, machinery, jigs and fixtures
- Coarse thread applications