M39 Grade 6.8 Torque Specification

Torque Values by Surface Condition

Stress area: 976.0 mm² · Proof load: 429,440 N · Clamp load (75%): 322,080 N

Surface Condition	K Factor	N·m	ft·lbs	in·lbs	kgf·cm
Plain/Dry	0.2	2512.2	1852.9	22234.9	25617.4
Oiled	0.15	1884.2	1389.7	16676.6	19213.6
Zinc Plated	0.18	2261.0	1667.6	20011.5	23055.9
Waxed	0.1	1256.1	926.5	11117.4	12808.7
MoS₂	0.12	1507.3	1111.7	13340.7	15370.2
Galvanized	0.25	3140.3	2316.2	27794.0	32022.3

Stress area: 1028.4 mm² · Proof load: 452,496 N · Clamp load (75%): 339,372 N

Surface Condition	K Factor	N·m	ft·lbs	in·lbs	kgf·cm
Plain/Dry	0.2	2647.1	1952.4	23428.8	26993.0
Oiled	0.15	1985.3	1464.3	17571.4	20244.5
Zinc Plated	0.18	2382.4	1757.2	21086.0	24293.8
Waxed	0.1	1323.6	976.2	11714.9	13497.0
MoS₂	0.12	1588.3	1171.5	14057.6	16196.2
Galvanized	0.25	3308.9	2440.5	29286.2	33741.5

Formula: T = K × D × F

Step 1 — Clamp load: F = 0.75 × 440 MPa × 976.0 mm² = 322,080 N

Step 2 — Torque (dry): T = 0.20 × 0.0390 m × 322,080 N = 2512.2 N·m

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

Industrial machinery, agricultural equipment, elevated load requirements

• 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

Class6.8

Diameter39 mm

Pitch (coarse)4.0 mm

Tensile600 MPa

Yield480 MPa

Proof440 MPa