#6 Grade 8 Bolt Torque: 1.9 ft·lbs / 2.6 N·m

Torque Values by Surface Condition

UNC — Coarse Thread — 32 TPI

Stress area: 0.00909 in² · Proof load: 1,091 lbs · Clamp load (75%): 818 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	1.88	2.55	22.6	26.0
Oiled	0.15	1.41	1.91	16.9	19.5
Zinc Plated	0.18	1.69	2.30	20.3	23.4
Waxed	0.1	0.94	1.28	11.3	13.0
MoS₂	0.12	1.13	1.53	13.5	15.6
Galvanized	0.25	2.35	3.19	28.2	32.5

UNF — Fine Thread — 40 TPI

Stress area: 0.01015 in² · Proof load: 1,218 lbs · Clamp load (75%): 913 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	2.10	2.85	25.2	29.0
Oiled	0.15	1.58	2.14	18.9	21.8
Zinc Plated	0.18	1.89	2.56	22.7	26.1
Waxed	0.1	1.05	1.42	12.6	14.5
MoS₂	0.12	1.26	1.71	15.1	17.4
Galvanized	0.25	2.63	3.56	31.5	36.3

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 0.94 ft·lbs (waxed) to 2.35 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 × 0.00909 in² = 818 lbs

Step 2 — Torque (dry): T = 0.20 × 0.138 in × 818 lbs ÷ 12 = 1.9 ft·lbs

Proof stress per SAE J429 Table 3. Stress area per ASME B1.1. Typical K-factor, consistent with VDI 2230 friction values. SAE J429 formally covers 1/4" to 1-1/2" diameters; for numbered machine screws these figures apply the same grade proof stress (as used for Grade 5/8 machine screws sold to SAE-equivalent strength) — check the screw supplier's rating.

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

Size#6

Grade8

Diameter0.138" (3.51 mm)

TPI (UNC)32

Tensile150,000 psi

Yield130,000 psi

Proof120,000 psi

#6 Grade 8 Torque Spec