1-3/8" Grade 8 Bolt Torque: 2382.2 ft·lbs

Torque Values by Surface Condition

UNC — Coarse Thread — 6 TPI

Stress area: 1.155 in² · Proof load: 138,600 lbs · Clamp load (75%): 103,950 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	2382.2	3229.8	28586.2	32934.9
Oiled	0.15	1786.6	2422.3	21439.7	24701.2
Zinc Plated	0.18	2144.0	2906.9	25727.6	29641.4
Waxed	0.1	1191.1	1614.9	14293.1	16467.5
MoS₂	0.12	1429.3	1937.9	17151.8	19761.0
Galvanized	0.25	2977.7	4037.2	35732.8	41168.7

UNF — Fine Thread — 12 TPI

Stress area: 1.315 in² · Proof load: 157,800 lbs · Clamp load (75%): 118,350 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	2712.2	3677.3	32546.2	37497.3
Oiled	0.15	2034.1	2757.9	24409.7	28123.0
Zinc Plated	0.18	2441.0	3309.6	29291.6	33747.6
Waxed	0.1	1356.1	1838.6	16273.1	18748.7
MoS₂	0.12	1627.3	2206.3	19527.8	22498.4
Galvanized	0.25	3390.2	4596.5	40682.8	46871.7

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 1191.1 ft·lbs (waxed) to 2977.7 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 × 1.155 in² = 103,950 lbs

Step 2 — Torque (dry): T = 0.20 × 1.375 in × 103,950 lbs ÷ 12 = 2382.2 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

Size1-3/8"

Grade8

Diameter1.375" (34.92 mm)

TPI (UNC)6

Tensile150,000 psi

Yield130,000 psi

Proof120,000 psi

1-3/8" Grade 8 Torque Spec