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#8 Grade 8 Torque Spec

Medium carbon alloy steel, quenched and tempered · Tensile 150,000 psi · Yield 130,000 psi

3.4
ft·lbs (dry)
4.6
N·m (dry)
2.6
ft·lbs (oiled)

Torque Values by Surface Condition

UNC — Coarse Thread — 32 TPI

Stress area: 0.014 in² · Proof load: 1,680 lbs · Clamp load (75%): 1,260 lbs

Surface Condition K Factor ft·lbs N·m in·lbs kgf·cm
Plain/Dry 0.2 3.44 4.67 41.3 47.6
Oiled 0.15 2.58 3.50 31.0 35.7
Zinc Plated 0.18 3.10 4.20 37.2 42.9
Waxed 0.1 1.72 2.33 20.7 23.8
MoS₂ 0.12 2.07 2.80 24.8 28.6
Galvanized 0.25 4.31 5.84 51.7 59.5

UNF — Fine Thread — 36 TPI

Stress area: 0.01474 in² · Proof load: 1,769 lbs · Clamp load (75%): 1,327 lbs

Surface Condition K Factor ft·lbs N·m in·lbs kgf·cm
Plain/Dry 0.2 3.63 4.92 43.5 50.1
Oiled 0.15 2.72 3.69 32.6 37.6
Zinc Plated 0.18 3.26 4.42 39.2 45.1
Waxed 0.1 1.81 2.46 21.8 25.1
MoS₂ 0.12 2.18 2.95 26.1 30.1
Galvanized 0.25 4.53 6.15 54.4 62.7

Torque by surface condition

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

Waxed (K = 0.1): 1.72 ft·lbsWaxedK = 0.11.72MoS₂ (K = 0.12): 2.07 ft·lbsMoS₂K = 0.122.07Oiled (K = 0.15): 2.58 ft·lbsOiledK = 0.152.58Zinc Plated (K = 0.18): 3.10 ft·lbsZinc PlatedK = 0.183.10Plain/Dry (K = 0.2): 3.44 ft·lbsPlain/DryK = 0.23.44Galvanized (K = 0.25): 4.31 ft·lbsGalvanizedK = 0.254.31

Same bolt, same target clamp load: thread friction alone moves the required torque from 1.72 ft·lbs (waxed) to 4.31 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 × 0.014 in² = 1,260 lbs

Step 2 — Torque (dry): T = 0.20 × 0.164 in × 1,260 lbs ÷ 12 = 3.4 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#8
Grade8
Diameter0.164" (4.17 mm)
TPI (UNC)32
Tensile150,000 psi
Yield130,000 psi
Proof120,000 psi

#8 — Other Grades

Grade 2 · 1.6 ft·lbsGrade 5 · 2.4 ft·lbs

Grade 8 — Other Sizes

#6 · 1.9 ft·lbs#10 · 5.0 ft·lbs1/4" · 11.9 ft·lbs5/16" · 24.6 ft·lbs3/8" · 43.6 ft·lbs1/2" · 106.4 ft·lbs3/4" · 375.8 ft·lbs1" · 909.0 ft·lbs1-1/8" · 1287.6 ft·lbs1-1/4" · 1816.9 ft·lbs1-3/8" · 2382.2 ft·lbs1-1/2" · 3161.3 ft·lbs

Explore

All #8 torque values → Torque calculator → ft·lbs ↔ N·m converter → Metric vs Imperial → Grade 5 vs Grade 8 → Automotive torque guide → Carbon steel torque → Nearest metric: M4 Class 10.9 →All Grade 8 sizes →Stainless bolt torque →in·lbs ↔ N·m converter →in·lbs ↔ ft·lbs converter →