M39 Grade 8.8 Torque Specification

Torque Values by Surface Condition

Stress area: 976.0 mm² · Proof load: 566,080 N · Clamp load (75%): 424,560 N

Surface Condition	K Factor	N·m	ft·lbs	in·lbs	kgf·cm
Plain/Dry	0.2	3311.6	2442.5	29310.1	33769.0
Oiled	0.15	2483.7	1831.9	21982.6	25326.8
Zinc Plated	0.18	2980.4	2198.2	26378.8	30391.7
Waxed	0.1	1655.8	1221.3	14655.1	16884.5
MoS₂	0.12	1986.9	1465.5	17585.6	20260.8
Galvanized	0.25	4139.5	3053.1	36637.7	42211.3

Stress area: 1028.4 mm² · Proof load: 596,472 N · Clamp load (75%): 447,354 N

Surface Condition	K Factor	N·m	ft·lbs	in·lbs	kgf·cm
Plain/Dry	0.2	3489.4	2573.6	30883.8	35582.1
Oiled	0.15	2617.0	1930.2	23162.4	26686.1
Zinc Plated	0.18	3140.4	2316.2	27794.9	32023.3
Waxed	0.1	1744.7	1286.8	15441.9	17791.1
MoS₂	0.12	2093.6	1544.2	18529.9	21348.9
Galvanized	0.25	4361.7	3217.0	38604.3	44477.1

Formula: T = K × D × F

Step 1 — Clamp load: F = 0.75 × 580 MPa × 976.0 mm² = 424,560 N

Step 2 — Torque (dry): T = 0.20 × 0.0390 m × 424,560 N = 3311.6 N·m

Proof stress per ISO 898-1:2013 Table 4. Stress area per ISO 898-1 Annex A. K-factor per VDI 2230.

Automotive engines, structural connections, high-strength machinery, flanged joints — the most common high-strength grade

• Torque values are for standard hex bolts with ISO metric threads. 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 (e.g., VDI 2230).
• Re-used bolts may have altered friction characteristics. Inspect threads before reuse.

SizeM39

Class8.8

Diameter39 mm

Pitch (coarse)4.0 mm

Tensile800 MPa

Yield640 MPa

Proof580 MPa