5/16" Grade 8 Bolt Torque: 24.6 ft·lbs / 33.4 N·m

Torque Values by Surface Condition

UNC — Coarse Thread — 18 TPI

Stress area: 0.0524 in² · Proof load: 6,288 lbs · Clamp load (75%): 4,716 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	24.6	33.4	294.8	339.6
Oiled	0.15	18.4	24.9	221.1	254.7
Zinc Plated	0.18	22.1	30.0	265.3	305.6
Waxed	0.1	12.3	16.7	147.4	169.8
MoS₂	0.12	14.7	19.9	176.8	203.8
Galvanized	0.25	30.7	41.6	368.4	424.5

UNF — Fine Thread — 24 TPI

Stress area: 0.058 in² · Proof load: 6,960 lbs · Clamp load (75%): 5,220 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	27.2	36.9	326.2	375.9
Oiled	0.15	20.4	27.7	244.7	281.9
Zinc Plated	0.18	24.5	33.2	293.6	338.3
Waxed	0.1	13.6	18.4	163.1	187.9
MoS₂	0.12	16.3	22.1	195.8	225.5
Galvanized	0.25	34.0	46.1	407.8	469.9

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 12.3 ft·lbs (waxed) to 30.7 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 × 0.0524 in² = 4,716 lbs

Step 2 — Torque (dry): T = 0.20 × 0.3125 in × 4,716 lbs ÷ 12 = 24.6 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

Size5/16"

Grade8

Diameter0.3125" (7.94 mm)

TPI (UNC)18

Tensile150,000 psi

Yield130,000 psi

Proof120,000 psi

5/16" Grade 8 Torque Spec