1/4-28 UNF Thread Specifications
Complete dimensions for 1/4-28 UNF (Unified National Fine) thread per ASME B1.1. TPI: 28, Pitch: 0.03571" (0.907 mm).
Thread Dimensions
| Designation | 1/4-28 UNF |
| Threads Per Inch (TPI) | 28 |
| Pitch | 0.03571" (0.907 mm) |
| Major Diameter | 0.25" (6.35 mm) |
| Pitch Diameter | 0.2268" (5.761 mm) |
| Minor Diameter (External) | 0.2062" (5.237 mm) |
| Minor Diameter (Internal) | 0.2113" (5.368 mm) |
| Thread Depth | 0.01933" (0.491 mm) |
| Tensile Stress Area | 0.03637 in² (23.5 mm²) |
Tap Drill Size
Recommended tap drill: #3 (0.213" / 5.41 mm)
Calculated minor diameter (internal): 0.2113" (5.37 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 | 2,000 | 2,691 |
| Grade 5 | 85,000 | 120,000 | 3,091 | 4,364 |
| Grade 8 | 120,000 | 150,000 | 4,364 | 5,456 |
Same Size, Different Series
Common Applications
- General purpose fastening, machinery, jigs and fixtures
- Fine thread applications