Bolt Torque Chart for Grade 8.8, 10.9 and 12.9 Bolts

Tightening torque is the turning force applied to a nut or bolt head. It stretches the bolt, and the stretched bolt clamps the joint together. This chart gives typical tightening torques for metric coarse thread high tensile bolts in property classes 8.8, 10.9 and 12.9, from M4 to M36, with the clamp load (preload) each torque produces.

Important: the torque given by the designer, the drawing or the equipment maker always comes first. Use this chart only where no torque is specified, as a general guide.

Bolt Torque Chart (N·m)

Typical tightening torque for a friction coefficient of about 0.14 (black or zinc plated bolts, lightly oiled), tightening each bolt to about 90 % of its minimum yield strength.

Basis: metric coarse thread, hex head bolts to ISO 4014 / ISO 4017 or socket head cap screws to ISO 4762, a nut or tapped hole of matching strength, clearance holes of the ISO 273 medium series, friction coefficient 0.14 in the thread and under the head. These are maximum typical values with no margin for over-tightening, so do not add extra.

Clamp Load (Preload) at These Torques (kN)

The clamp load is what actually holds the joint. Even with a calibrated torque wrench it can vary by 20 % or more either way, because friction varies from bolt to bolt.

How the Values Are Calculated

The clamp load is the largest load at which the combined tension and twisting stress in the thread reaches 90 % of the minimum yield strength given in ISO 898-1: 640 MPa for 8.8 up to M16, 660 MPa for 8.8 above M16, 940 MPa for 10.9 and 1100 MPa for 12.9. The stress is taken on the stress area of the thread, the same area ISO 898-1 uses for bolt strength.

The torque is the clamp load multiplied by a factor for the thread pitch (P), the thread friction and the friction under the head: torque = clamp load × (0.16 × P + 0.58 × d2 × μ + 0.5 × Dkm × μ), where d2 is the pitch diameter, μ the friction coefficient and Dkm the mean diameter of the bearing face under the head. With the clamp load in kN and the sizes in mm, the result is in N·m. This is the method of the VDI 2230 guideline, and the results agree with the usual VDI 2230 tables for friction 0.14 to within about 1 %.

How Friction Changes the Torque

Only about 10 to 15 % of the torque stretches the bolt. The other 85 to 90 % is used up by friction in the thread and under the head, so a small change in friction makes a big change in clamp load.

• Dry threads: unoiled bolts and nuts have higher friction, often 0.2 or more. The table torque then gives roughly a quarter less clamp load, and the joint may work loose.
• Oiled, greased or waxed threads: friction can fall to about 0.10, or lower with molybdenum disulphide paste. The same torque then gives about a third more clamp load and can stretch the bolt past its yield point or break it. Lower the torque when you lubricate, unless the specified torque is meant for lubricated bolts.
• Hot dip galvanized (HDG) bolts: the thick zinc coating gives high and uneven friction, and dry threads can seize. Lubricate the nut thread and use the torque or tightening method given for the job. High strength friction grip (HSFG) bolts in steel structures are tightened by the method in the structural code, for example IS 4000, not from a general chart.
• Stainless steel bolts: SS 304 and SS 316 threads can gall, which means the thread surfaces weld together and seize, when they are tightened dry or fast. Use an anti-seize compound and tighten slowly and steadily. Do not use this chart for stainless steel: A2-70 and A4-70 bolts have a minimum yield strength of 450 MPa against 640 MPa for 8.8, so they need a lower torque. Take the torque from the designer or the stainless fastener supplier. Our SS 304 vs SS 316 guide explains the stainless grades.

Tips for Correct Tightening

• Use a calibrated torque wrench and tighten in one smooth, steady pull.
• Use a nut of the matching class: class 8 for 8.8 bolts, class 10 for 10.9 and class 12 for 12.9 (ISO 898-2). See the bolt grade chart.
• On flanges and plates with several bolts, tighten in a cross pattern in two or three passes.
• Use the right spanner or socket so the head is not damaged. See the spanner size chart.
• Replace bolts with damaged or stretched threads.

High Tensile Bolts from JMIKS Fasteners

JMIKS Fasteners supplies high tensile bolts and fasteners in grades 8.8, 10.9 and 12.9: hex bolts, Allen cap screws, Allen CSK screws, button head screws and grub screws, with matching hex nuts. Hex bolts and Allen cap screws run from M3 to M36. Mild steel and high tensile fasteners come in self colour, zinc plated and hot dip galvanized finishes, and grade 12.9 Allen cap screws are usually supplied black (self colour). See the full bolts range.

Bolt Torque FAQs

What torque should I use for an M12 8.8 bolt?

About 93 N·m at a friction coefficient of 0.14 (black or zinc plated, lightly oiled). This gives a clamp load of about 42 kN. If the drawing or the equipment manual gives a torque, use that instead.

What is the torque for M16 bolts in 8.8, 10.9 and 12.9?

At friction 0.14: about 230 N·m for 8.8, 338 N·m for 10.9 and 395 N·m for 12.9. The clamp loads are about 79 kN, 116 kN and 136 kN.

Should bolts be oiled before tightening?

Only if the specified torque is for oiled bolts. Oil lowers friction, so the same torque gives more clamp load: about a third more when friction falls from 0.14 to 0.10. A torque meant for dry or lightly oiled bolts can overload a well greased bolt.

Can I use this chart for stainless steel bolts?

No. SS 304 (A2) and SS 316 (A4) bolts have a lower yield strength than 8.8 bolts, and their threads can gall. Use the torque from the designer or the stainless fastener supplier, with an anti-seize compound.

Does the chart apply to Allen cap screws?

Yes, for socket head cap screws to ISO 4762 / DIN 912 of the same grade, because the bearing area under the head is similar to a hex bolt. Countersunk and button head socket screws have a smaller socket and a weaker head, so they need a lower torque.

Get a Quote for High Tensile Bolts

JMIKS Fasteners is a wholesaler and supplier of high tensile, mild steel and stainless steel fasteners in Nava Odhav, Ahmedabad. Send the size, grade, finish and quantity on WhatsApp (+91 90333 11425) or through the enquiry form, or download the catalogue (PDF). We reply within one working day.