Metric Bolt Torque Chart: Tightening Torques for Class 8.8, 10.9 & 12.9

Why Tightening Torque Matters

A bolt is a spring. Its job is to stretch slightly and clamp your parts together with a controlled preload — typically 65–75% of the bolt’s proof load. What makes torque tricky is friction: roughly 90% of the torque you apply is consumed by friction under the bolt head and in the threads. Only about 10% actually stretches the bolt. That is why the same torque value produces different clamping forces on dry, lubricated, or galvanized bolts.

Under-torque a bolt and the joint loosens under vibration; over-torque it and you yield the bolt or strip the thread. The chart below gives the standard recommended values for metric hex bolts.

Metric Bolt Torque Chart – Class 8.8, 10.9, 12.9

Recommended tightening torques in newton-metres (Nm) for metric coarse-thread hex bolts, dry, zinc-plated, with a standard flat washer:

Bolt Size	Pitch (mm)	Class 8.8	Class 10.9	Class 12.9
M6	1.00	9.5	13.5	16.0
M8	1.25	23	33	39
M10	1.50	46	65	78
M12	1.75	79	112	135
M14	2.00	126	179	215
M16	2.00	195	275	330
M18	2.50	270	385	460
M20	2.50	385	545	655
M22	2.50	520	735	880
M24	3.00	665	940	1,130
M27	3.00	990	1,400	1,680
M30	3.50	1,300	1,850	2,220
M36	4.00	2,250	3,190	3,830

Values are indicative assembly torques targeting approximately 70% of proof load, and assume unlubricated zinc-plated bolts with a standard washer. For structural or safety-critical joints, always follow the engineer’s specification or calculate to VDI 2230.

What Changes the Right Torque Value

The relationship between torque and clamping force is expressed as T = K × F × d, where T is torque, F is the target preload, d is the nominal diameter, and K is the friction-dependent “nut factor”. Everything that changes friction changes K:

Lubrication: oiled or waxed threads lower friction by 20–30%, so the same torque produces MORE preload. Reduce torque accordingly — never re-use the dry-torque value on lubricated bolts.
Hot-dip galvanizing: the thick zinc coating increases and varies friction. Galvanized structural bolts should be lubricated (typically with wax) and tightened to the values specified by the coating system, not the black-bolt chart.
Stainless steel: A2/A4 bolts gall easily. Apply an anti-seize or wax lubricant and target torque values comparable to class 8.8 — stainless has no head-marked class, and A2-70 strength is roughly similar to 8.8.
Washers: a hardened flat washer under the turning element stabilizes friction and protects the surface, making the torque reading more repeatable.

Torque Wrench Accuracy in Practice

A click-type torque wrench is accurate to about ±4%; a beam type to ±8%, and an uncalibrated one to anything. For production lines, calibrated wrenches should be checked at least annually, and for high-strength structural joints, engineers increasingly prefer direct tension methods — tension control (TC) bolts break off the spline at a calibrated tension rather than relying on torque at all, and high-strength friction-grip (HSFG) structural bolts use a combined torque-plus-turn procedure.

When Torque Charts Are Not Enough

Torque is an indirect, friction-dependent estimate of preload. For joints where clamp load is critical — flanges, foundation anchors, lifting hardware — specify the required preload and control it directly: TC bolts, hydraulic tensioners, or angle-controlled tightening. We supply TC bolts and HSFG sets with the matching certification; ask for EN 10204 3.1 mill test certificates on any order.

Frequently Asked Questions

Can I use class 10.9 torque values on a stainless bolt?

No. A2-70 stainless steel is roughly comparable to class 8.8 in strength, so use the 8.8 column at most — and lubricate the threads to prevent galling. Over-torquing stainless bolts seizes or snaps them.

Do hot-dip galvanized bolts need different torque?

Yes. The zinc layer raises and varies the friction coefficient. Use the manufacturer’s lubricated-thread values (commonly 10–15% lower than dry black-bolt torque) and never assemble HDG bolts dry if you can avoid it.

Should I re-tighten bolts after installation?

For most static joints, no. Some standards (notably structural steel after initial settlement) call for a check pass at 1–7 days. Follow your project spec; arbitrary re-tightening can over-yield the bolt.