#8 Screw Torque
An #8 A574 screw with a coarse 8-32 UNC thread needs about 50.1 in·lb (5.66 N·m) when dry and 37.9 in·lb (4.29 N·m) when oiled, tightened to 70% of yield (clamp force 6.54 kN).
| Grade | Clamp forcepreload | Dryμ 0.15 | Oiledμ 0.11 | PTFE / moly pasteμ 0.06 |
|---|---|---|---|---|
| A574 | 6.54 kN1471 lbf | 50.1 in·lb5.66 N·m | 37.9 in·lb4.29 N·m | 25.8 in·lb2.91 N·m |
| Grade | Clamp forcepreload | Dryμ 0.15 | Oiledμ 0.11 | PTFE / moly pasteμ 0.06 |
|---|---|---|---|---|
| A574 | 6.88 kN1546 lbf | 52.1 in·lb5.88 N·m | 39.2 in·lb4.43 N·m | 26.4 in·lb2.98 N·m |
Torque for #8 screws with a cap-screw head (head size estimated) on steel with a 5.0 mm clearance hole. Friction μ applies to both the threads and the bearing face. Always follow the manufacturer's value where one exists.
#8 thread and dimensions
- Coarse thread
- 8-32 UNC — pitch 32 TPI
- Fine thread
- 8-36 UNF — pitch 36 TPI
- Tensile stress area Aₛ
- 9.04 mm² (coarse), 9.50 mm² (fine)
- Pitch diameter d₂
- 3.650 mm
- Tap drill
- #29 (coarse), #29 (fine)
- Clearance hole
- 5.0 mm
How the #8 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 × 9.04 mm² = 6.54 kN
T = F × (0.16·P + 0.58·μ·d₂ + μ·Dₖₘ/2)
T = 6.54 kN × (0.16·0.794 + 0.58·0.15·3.65 + 0.15·5.81/2) mm = 5.66 N·m
Here d₂ is the pitch diameter and Dₖₘ the effective bearing diameter of the head (5.81 mm). Change the grade, lubrication or preload in the calculator below.
Frequently asked questions
What is the torque for an #8 A574 screw?
About 50.1 in·lb (5.66 N·m) with dry threads, 37.9 in·lb (4.29 N·m) with oiled threads and 25.8 in·lb (2.91 N·m) with PTFE or moly paste, for a coarse 8-32 UNC thread tightened to 70% of yield. The resulting clamp force is 6.54 kN.
What drill size do I need to tap #8?
Use a #29 drill for 8-32 UNC and #29 for 8-36 UNF.