M8 bolt torque specifications (8.8, 10.9, 12.9)
An M8 x 1.25 class 8.8 bolt takes 19 ft-lb (25 N·m) dry, and class 10.9 takes 27 ft-lb (36 N·m). Both are for plain steel, dry threads (K 0.20) tightened to a clamp load of 75% of proof load.
Contents14 sections
Key takeaways
- Class 8.8 dry: 19 ft-lb (25 N·m); class 10.9 dry: 27 ft-lb (36 N·m).
- The 8.8 clamp load is 15.9 kN, 75% of the 21,200 N proof load ISO 898-1 lists for M8 1.
- M8 x 1 fine carries about 7% more clamp load than M8 x 1.25.
- An 8 mm spark plug is not an M8 bolt: NGK lists 5.8 to 7.2 ft-lb in aluminum heads 2.
Quick facts
| Coarse thread | M8 x 1.25 (36.6 mm²) |
|---|---|
| Fine thread | M8 x 1 (39.2 mm²) |
| Wrench, ISO 4014/4017 | 13 mm |
| Wrench, DIN 931/933 | 13 mm (same) |
M8 bolt specifications
| Thread | Pitch | Tensile stress area |
|---|---|---|
| M8 x 1.25 (coarse) | 1.25 mm pitch | 36.6 mm² |
| M8 x 1 (fine) | 1 mm pitch | 39.2 mm² |
| Wrench, ISO 4014/4017 | 13 mm3 | |
Stress areas from ISO 898-1 Table 41.
M8 torque by grade
| Grade | Plain, dry K 0.203 | Lubricated K 0.153 | Clamp load |
|---|---|---|---|
| Class 4.6 | 87 in-lb9.9 N·m | 66 in-lb7.4 N·m | 6.2 kN |
| Class 8.8 | 19 ft-lb25 N·m | 14 ft-lb19 N·m | 15.9 kN |
| Class 10.9 | 27 ft-lb36 N·m | 20 ft-lb27 N·m | 22.8 kN |
| Class 12.9 | 31 ft-lb43 N·m | 24 ft-lb32 N·m | 26.6 kN |
Calculated with T = K × D × F, where F is 75% of the bolt’s proof load3 and K is the nut factor for each condition. Real preload from a torque wrench scatters by about ±25%4. When the equipment maker gives a torque, use theirs. How these numbers are made.
M8 4.6 torque
Proof 225 MPa · yield 240 MPa · tensile 400 MPa1. Clamp load at 75% of proof: 6.2 kN (M8 x 1.25).
| Condition | K | M8 x 1.25 | M8 x 1 |
|---|---|---|---|
| Plain steel, dryAs-received black or plain finish, clean threads, no lubricant | 0.203 | 87 in-lb9.9 N·m | 94 in-lb11 N·m |
| Zinc-plated, dryClean zinc-plated threads, no added lubricant | 0.183 | 79 in-lb8.9 N·m | 84 in-lb9.5 N·m |
| Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head | 0.153 | 66 in-lb7.4 N·m | 70 in-lb7.9 N·m |
| Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize | 0.165 | 70 in-lb7.9 N·m | 75 in-lb8.5 N·m |
| Moly pasteMolybdenum disulfide paste | 0.116 | 48 in-lb5.4 N·m | 52 in-lb5.8 N·m |
M8 8.8 torque
Proof 580 MPa · yield 640 MPa · tensile 800 MPa1. Clamp load at 75% of proof: 15.9 kN (M8 x 1.25).
| Condition | K | M8 x 1.25 | M8 x 1 |
|---|---|---|---|
| Plain steel, dryAs-received black or plain finish, clean threads, no lubricant | 0.203 | 19 ft-lb25 N·m | 20 ft-lb27 N·m |
| Zinc-plated, dryClean zinc-plated threads, no added lubricant | 0.183 | 17 ft-lb23 N·m | 18 ft-lb25 N·m |
| Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head | 0.153 | 14 ft-lb19 N·m | 15 ft-lb20 N·m |
| Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize | 0.165 | 15 ft-lb20 N·m | 16 ft-lb22 N·m |
| Moly pasteMolybdenum disulfide paste | 0.116 | 10 ft-lb14 N·m | 11 ft-lb15 N·m |
M8 10.9 torque
Proof 830 MPa · yield 940 MPa · tensile 1040 MPa1. Clamp load at 75% of proof: 22.8 kN (M8 x 1.25).
| Condition | K | M8 x 1.25 | M8 x 1 |
|---|---|---|---|
| Plain steel, dryAs-received black or plain finish, clean threads, no lubricant | 0.203 | 27 ft-lb36 N·m | 29 ft-lb39 N·m |
| Zinc-plated, dryClean zinc-plated threads, no added lubricant | 0.183 | 24 ft-lb33 N·m | 26 ft-lb35 N·m |
| Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head | 0.153 | 20 ft-lb27 N·m | 22 ft-lb29 N·m |
| Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize | 0.165 | 22 ft-lb29 N·m | 23 ft-lb31 N·m |
| Moly pasteMolybdenum disulfide paste | 0.116 | 15 ft-lb20 N·m | 16 ft-lb21 N·m |
M8 12.9 torque
Proof 970 MPa · yield 1100 MPa · tensile 1220 MPa1. Clamp load at 75% of proof: 26.6 kN (M8 x 1.25).
| Condition | K | M8 x 1.25 | M8 x 1 |
|---|---|---|---|
| Plain steel, dryAs-received black or plain finish, clean threads, no lubricant | 0.203 | 31 ft-lb43 N·m | 34 ft-lb46 N·m |
| Zinc-plated, dryClean zinc-plated threads, no added lubricant | 0.183 | 28 ft-lb38 N·m | 30 ft-lb41 N·m |
| Lubricated (oil or grease)Light oil or grease on the threads and under the nut or head | 0.153 | 24 ft-lb32 N·m | 25 ft-lb34 N·m |
| Copper or metal-free anti-seizeLoctite C5-A copper or N-7000 metal-free anti-seize | 0.165 | 25 ft-lb34 N·m | 27 ft-lb37 N·m |
| Moly pasteMolybdenum disulfide paste | 0.116 | 17 ft-lb23 N·m | 19 ft-lb25 N·m |
Coarse or fine pitch
M8 x 1.25 coarse has a stress area of 36.6 mm² 1; M8 x 1 fine has 39.2 mm² 3, about 7% more. In class 8.8 dry that is 20 ft-lb instead of 19 ft-lb, with 17.1 kN of clamp load instead of 15.9 kN. A second fine pitch, M8 x 0.75, appears in the ISO 262 table 7; by the ISO stress-area formula it has about 41.8 mm², some 14% more than the coarse thread. It is not in the tables above, so calculate it with the bolt torque calculator.
Where the numbers come from at M8
ISO 898-1 tabulates the M8 coarse proof load as 21,200 N for class 8.8, 30,400 N for 10.9 and 35,500 N for 12.9 1. Taking 75% of each gives the clamp loads behind the chart: 15.9 kN (about 3,580 lbf) for 8.8 and 22.8 kN (about 5,120 lbf) for 10.9.
M8 is the first size where class 8.8 dry passes 10 ft-lb. M6 8.8 is 93 in-lb; M8 needs about 2.4 times that.
M8 next to 5/16 inch
M8 (8 mm) sits close to 5/16 in (7.94 mm). Calculated on the same basis, an M8 x 1.25 class 8.8 takes about 8% more torque than a 5/16-18 Grade 5 and clamps about 7% more. The threads do not interchange, so pick the table by the thread.
Wrench size
An M8 hex head takes a 13 mm wrench in both ISO 4014/4017 and DIN 931/933 3. The ISO and DIN sizes first differ one size up, at M10.
Which class you have
The property class (8.8, 10.9 or 12.9) is marked on the head with the maker’s mark 1; see the bolt grades page. Class 12.9 dry is 31 ft-lb (43 N·m), about 17% above 10.9.
When the chart does not apply
Spark plugs with an 8 mm thread seal on a gasket and are torqued far lower than a bolt: NGK gives 5.8 to 7.2 ft-lb for a flat-seat 8 mm plug in an aluminum head 2, against 19 ft-lb for an 8.8 bolt. See spark plug torque. Oiled M8 class 8.8 bolts take 14 ft-lb (19 N·m) at K 0.15 3; see dry vs lubricated torque.
Sources
- 1
- 2NGK Spark Plugs (Niterra), How do I install spark plugs correctly?Checked 21 Oct 2026
- 3
- 4
- 5
- 6Henkel Corporation (LOCTITE), LOCTITE LB 8012 Moly Paste TDS, April 2015Checked 21 Oct 2026
- 7Wikipedia, Wikipedia: ISO metric screw thread (ISO 262 selected sizes table)Checked 21 Oct 2026
Spotted a value that doesn’t match its source? Report an error
Related
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- M10M10 x 1.5 bolt torque: 37 ft-lb (50 N·m) for class 8.8 and 53 ft-lb (72 N·m) for 10.9 dry, plus 12.9, M10 x 1.25 fine and the 16 vs 17 mm wrench.
- 5/16 in5/16-18 Grade 5 bolt torque is 17 ft-lb dry and Grade 8 is 25 ft-lb. Charts for 5/16-18 and 5/16-24 in Grade 2, 5 and 8, dry, zinc-plated and oiled.
- Class 8.8Class 8.8 bolt torque from M3 to M30, dry, zinc-plated and lubricated, with the ISO 898-1 proof stress step from 580 MPa to 600 MPa above M16.
- Class 10.9Class 10.9 bolt torque from M3 to M30, dry, zinc-plated and lubricated, with ISO 898-1 strength (830 MPa proof) and how it compares with 8.8 and 12.9.
- Bolt torque calculatorBolt torque calculator for SAE and metric bolts by size, thread, grade and lubrication, using T = K × D × F with the proof loads from SAE J429 and ISO 898-1.
- Dry vs lubricated torqueWhy oiled bolts need less torque than dry ones for the same clamp load, with nut factor K values from Fastenal, NASA and Henkel for oil, zinc, anti-seize and moly.