#4 Screw Torque
A #4 A574 screw with a coarse 4-40 UNC thread needs about 15.2 in·lb (1.72 N·m) when dry and 11.6 in·lb (1.31 N·m) when oiled, tightened to 70% of yield (clamp force 2.82 kN).
| Grade | Clamp forcepreload | Dryμ 0.15 | Oiledμ 0.11 | PTFE / moly pasteμ 0.06 |
|---|---|---|---|---|
| A574 | 2.82 kN633 lbf | 15.2 in·lb1.72 N·m | 11.6 in·lb1.31 N·m | 8.00 in·lb0.90 N·m |
| Grade | Clamp forcepreload | Dryμ 0.15 | Oiledμ 0.11 | PTFE / moly pasteμ 0.06 |
|---|---|---|---|---|
| A574 | 3.08 kN693 lbf | 16.3 in·lb1.85 N·m | 12.4 in·lb1.40 N·m | 8.37 in·lb0.95 N·m |
Torque for #4 screws with a cap-screw head (head size estimated) on steel with a 3.6 mm clearance hole. Friction μ applies to both the threads and the bearing face. Always follow the manufacturer's value where one exists.
#4 thread and dimensions
- Coarse thread
- 4-40 UNC — pitch 40 TPI
- Fine thread
- 4-48 UNF — pitch 48 TPI
- Tensile stress area Aₛ
- 3.89 mm² (coarse), 4.26 mm² (fine)
- Pitch diameter d₂
- 2.432 mm
- Tap drill
- #43 (coarse), #42 (fine)
- Clearance hole
- 3.6 mm
How the #4 torque is calculated
Preload is 70% of the bolt's yield strength times its tensile stress area. Torque then follows the ISO 16047 / VDI 2230 formula, which splits it into thread pitch, thread friction and friction under the head:
F = 70% × 1034 MPa × 3.89 mm² = 2.82 kN
T = F × (0.16·P + 0.58·μ·d₂ + μ·Dₖₘ/2)
T = 2.82 kN × (0.16·0.635 + 0.58·0.15·2.43 + 0.15·4.10/2) mm = 1.72 N·m
Here d₂ is the pitch diameter and Dₖₘ the effective bearing diameter of the head (4.10 mm). Change the grade, lubrication or preload in the calculator below.
Frequently asked questions
What is the torque for a #4 A574 screw?
About 15.2 in·lb (1.72 N·m) with dry threads, 11.6 in·lb (1.31 N·m) with oiled threads and 8.00 in·lb (0.90 N·m) with PTFE or moly paste, for a coarse 4-40 UNC thread tightened to 70% of yield. The resulting clamp force is 2.82 kN.
What drill size do I need to tap #4?
Use a #43 drill for 4-40 UNC and #42 for 4-48 UNF.