#8 Grade 2 Bolt Torque: 1.6 ft·lbs / 2.2 N·m

Torque Values by Surface Condition

UNC — Coarse Thread — 32 TPI

Stress area: 0.014 in² · Proof load: 770 lbs · Clamp load (75%): 578 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	1.58	2.14	18.9	21.8
Oiled	0.15	1.18	1.61	14.2	16.4
Zinc Plated	0.18	1.42	1.93	17.0	19.6
Waxed	0.1	0.79	1.07	9.5	10.9
MoS₂	0.12	0.95	1.28	11.4	13.1
Galvanized	0.25	1.97	2.68	23.7	27.3

UNF — Fine Thread — 36 TPI

Stress area: 0.01474 in² · Proof load: 811 lbs · Clamp load (75%): 608 lbs

Surface Condition	K Factor	ft·lbs	N·m	in·lbs	kgf·cm
Plain/Dry	0.2	1.66	2.25	19.9	23.0
Oiled	0.15	1.25	1.69	15.0	17.2
Zinc Plated	0.18	1.50	2.03	17.9	20.7
Waxed	0.1	0.83	1.13	10.0	11.5
MoS₂	0.12	1.00	1.35	12.0	13.8
Galvanized	0.25	2.08	2.82	24.9	28.7

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 0.79 ft·lbs (waxed) to 1.97 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 × 55,000 psi × 0.014 in² = 578 lbs

Step 2 — Torque (dry): T = 0.20 × 0.164 in × 578 lbs ÷ 12 = 1.6 ft·lbs

Proof stress per SAE J429 Table 3. Stress area per ASME B1.1. Typical K-factor, consistent with VDI 2230 friction values. 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 2

General-purpose, non-critical bolting. Most common ungraded hardware-store bolts.

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

Grade2

Diameter0.164" (4.17 mm)

TPI (UNC)32

Tensile74,000 psi

Yield57,000 psi

Proof55,000 psi

#8 Grade 2 Torque Spec