Class 8.8 bolt torque chart and strength
An M12 x 1.75 class 8.8 bolt takes 65 ft-lb (88 N·m) plain and dry or 49 ft-lb (66 N·m) oiled, for 36.7 kN of clamp load. Proof stress is 580 MPa up to M16 and 600 MPa above.
Contents9 sections
Key takeaways
Quick facts
| Proof, M16 and smaller | 580 MPa |
|---|---|
| Proof, over M16 | 600 MPa |
| Head marking | 8.8 plus maker's mark |
| Inch near-equivalent | Grade 5 |
Class 8.8 strength
| Sizes | Proof stress | Yield | Tensile |
|---|---|---|---|
| M16 and smaller | 580 MPa | 640 MPa | 800 MPa |
| Over M16 | 600 MPa | 660 MPa | 830 MPa |
ISO 898-1 Table 31.
Class 8.8 torque chart
| Size | Dry K 0.202 | Zinc K 0.182 | Oiled K 0.152 |
|---|---|---|---|
| M3 x 0.5 | 12 in-lb1.3 N·m | 10 in-lb1.2 N·m | 9 in-lb1.0 N·m |
| M4 x 0.7 | 27 in-lb3.1 N·m | 24 in-lb2.7 N·m | 20 in-lb2.3 N·m |
| M5 x 0.8 | 55 in-lb6.2 N·m | 49 in-lb5.6 N·m | 41 in-lb4.6 N·m |
| M6 x 1 | 93 in-lb10 N·m | 84 in-lb9.4 N·m | 70 in-lb7.9 N·m |
| M8 x 1.25 | 19 ft-lb25 N·m | 17 ft-lb23 N·m | 14 ft-lb19 N·m |
| M10 x 1.5 | 37 ft-lb50 N·m | 33 ft-lb45 N·m | 28 ft-lb38 N·m |
| M12 x 1.75 | 65 ft-lb88 N·m | 58 ft-lb79 N·m | 49 ft-lb66 N·m |
| M14 x 2 | 103 ft-lb140 N·m | 93 ft-lb126 N·m | 77 ft-lb105 N·m |
| M16 x 2 | 161 ft-lb219 N·m | 145 ft-lb197 N·m | 121 ft-lb164 N·m |
| M20 x 2.5 | 325 ft-lb441 N·m | 293 ft-lb397 N·m | 244 ft-lb331 N·m |
| M24 x 3 | 562 ft-lb762 N·m | 506 ft-lb686 N·m | 422 ft-lb572 N·m |
| M30 x 3.5 | 1,117 ft-lb1,515 N·m | 1,005 ft-lb1,363 N·m | 838 ft-lb1,136 N·m |
Calculated with T = K × D × F, where F is 75% of the bolt’s proof load2 and K is the nut factor for each condition. Real preload from a torque wrench scatters by about ±25%3. When the equipment maker gives a torque, use theirs. How these numbers are made.
What 8.8 means
The first 8 is the nominal tensile strength divided by 100 MPa. The second 8 is ten times the yield-to-tensile ratio 1. Class 8.8 is therefore 800 MPa nominal, with yield at 80% of that. ISO 898-1 covers carbon and alloy steel bolts 1. Its material and heat-treatment table is not in the free preview this site used, so it is not summarized here.
The head carries “8.8” and the maker’s identification mark. A leading zero, “08.8”, marks reduced loadability 1.
The M16 strength break
ISO 898-1 gives 8.8 two sets of minimums 1:
- M16 and smaller: 580 MPa proof, 640 MPa yield, 800 MPa tensile.
- Over M16: 600 MPa proof, 660 MPa yield, 830 MPa tensile.
The step is 3.4%, and it is applied from M20 up on this site. Charts that use one 8.8 strength for all sizes, or label a range “16 mm and up”, are ambiguous at M16. M16 uses the lower value here: 161 ft-lb (219 N·m) dry.
There are two notes for structural bolting. The M16-and-smaller values do not apply, and the over-16 mm column applies from M12 1. For structural work, follow the structural specification, not this table.
How the numbers are built
ISO tabulates the proof load of an M12 8.8 bolt as 48,900 N 1. Taking 75% of that gives the 36.7 kN clamp load used for every condition on the chart 2. Dry (K 0.20) needs 88 N·m, zinc-plated (K 0.18) 79 N·m, and oiled (K 0.15) 66 N·m. Published M12 8.8 values range from about 74 to 100 N·m because charts pick different clamp targets and friction values. See the metric chart for why.
Compared with other classes
At the same size and condition:
- 10.9: 830 ÷ 580, about 43% more, up to M16. About 38% more above M16.
- 12.9: about 67% more up to M16, and 62% more above.
- 4.6: about 39% of the 8.8 value up to M16, and 38% above.
- SAE Grade 5: 85 ksi proof is 586 MPa, within 1% of 8.8’s 580 MPa 4. The threads are not interchangeable.
Matching nut
Use a nut whose proof strength is at least the bolt’s tensile strength 2. In the older ISO 898-2 table, class 8 nuts are rated at 880 MPa from M10 to M16 and 920 MPa from M16 to M39 2. Both cover 8.8’s 800 and 830 MPa tensile strengths. A class 8 nut meets the rule, and a class 10 nut has more margin.
Sources
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- 4
Spotted a value that doesn’t match its source? Report an error
Related
- 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.
- Grade 5Grade 5 bolt torque from 1/4 in to 1 in, dry, zinc-plated and lubricated, with SAE J429 strength, the three-line head mark and the step down to 74 ksi above 1 in.
- M12M12 x 1.75 bolt torque: 65 ft-lb (88 N·m) for class 8.8 and 93 ft-lb (126 N·m) for 10.9 dry, plus 12.9, M12 x 1.25 fine and the 18 vs 19 mm wrench.
- Metric chartMetric bolt torque chart for classes 4.6, 8.8, 10.9 and 12.9 from M3 to M30, dry and lubricated, in N·m and ft-lb from ISO 898-1 proof loads.