3/8" Grade 8 Bolt Torque: 43.6 ft·lbs / 59.1 N·m

Torque Values by Surface Condition

UNC — Coarse Thread — 16 TPI

Stress area: 0.0775 in² · Proof load: 9,300 lbs · Clamp load (75%): 6,975 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	43.6	59.1	523.1	602.7
Oiled	0.15	32.7	44.3	392.3	452.0
Zinc Plated	0.18	39.2	53.1	470.8	542.4
Waxed	0.1	21.8	29.6	261.6	301.4
MoS₂	0.12	26.2	35.5	313.9	361.6
Galvanized	0.25	54.5	73.9	653.9	753.4

UNF — Fine Thread — 24 TPI

Stress area: 0.0878 in² · Proof load: 10,536 lbs · Clamp load (75%): 7,902 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	49.4	67.0	592.7	682.8
Oiled	0.15	37.0	50.2	444.5	512.1
Zinc Plated	0.18	44.4	60.2	533.4	614.5
Waxed	0.1	24.7	33.5	296.3	341.4
MoS₂	0.12	29.6	40.1	355.6	409.7
Galvanized	0.25	61.7	83.7	740.8	853.5

Torque by surface condition

UNC thread, ft·lbs. Values from the table above.

Same bolt, same target clamp load: thread friction alone moves the required torque from 21.8 ft·lbs (waxed) to 54.5 ft·lbs (hot-dip galvanized), 2.5× as much. Match the row to the fastener's actual finish and lubrication.

How This Value Is Calculated

Formula: T = K × D × F

Step 1 — Clamp load: F = 0.75 × 120,000 psi × 0.0775 in² = 6,975 lbs

Step 2 — Torque (dry): T = 0.20 × 0.375 in × 6,975 lbs ÷ 12 = 43.6 ft·lbs

Proof stress per SAE J429 Table 3. Stress area per ASME B1.1. Typical K-factor, consistent with VDI 2230 friction values.

When to Use Grade 8

High-strength applications, heavy equipment, critical joints, suspension components, trailer hitches. Equivalent to ISO 10.9.

Important Notes

• Torque values are for standard hex bolts with unified threads per ASME B1.1. Socket head cap screws may have different requirements.
• Lubrication reduces required torque significantly — using dry torque values on lubricated bolts risks exceeding yield strength.
• For critical or safety-related applications, always consult the manufacturer's specifications or applicable design standard.
• Re-used bolts may have altered friction characteristics. Inspect threads before reuse.

Specifications

Size3/8"

Grade8

Diameter0.375" (9.52 mm)

TPI (UNC)16

Tensile150,000 psi

Yield130,000 psi

Proof120,000 psi

3/8" Grade 8 Torque Spec