Tightening torques for stainless steel bolts
Introduction
Minimum breaking torque, (MB, min), for austenitic grade bolts and screws between M 1.6 and M 16, (coarse threads), are shown in Table 8 of BS EN ISO 3506-1:2020. This shows torque values in Nm for property classes 50, 70 and 80 as shown in the table below. This applies to ‘grades’ A1, A2, A3, A4 and A5. Tightening torque values are not given in this standard.
Minimum breaking torque values
Torque values are not given for martensitic, (C1, C3, C4), or ferritic, (F1), grades. The standard states that these values are to be agreed between manufacturer and user.
| Thread |
Min breaking torque, MB, Nm, for property class |
| 50 |
70 |
80 |
| M1.6 |
0.15 |
0.2 |
0.24 |
| M2 |
0.3 |
0.4 |
0.48 |
| M2.5 |
0.6 |
0.9 |
0.96 |
| M3 |
1.1 |
1.6 |
1.8 |
| M4 |
2.7 |
3.8 |
4.3 |
| M5 |
5.5 |
7.8 |
8.8 |
| M6 |
9.3 |
13 |
15 |
| M8 |
23 |
32 |
37 |
| M10 |
46 |
65 |
74 |
| M12 |
80 |
110 |
130 |
| M16 |
210 |
290 |
330 |
Maximum tightening torque, yield loads and failure loads for austenitic stainless steel fasteners
Tightening torques, yield loads and failure loads are not given in the BSENISO 3506-1:1997 standard. Some data is available however on the Bufab UK web site.
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