The number stamped on a hex head — 8.8, 10.9, 12.9 — is a property class, not a quality rating. It encodes two mechanical properties, and understanding both is what keeps you from specifying an upgrade that quietly makes the joint weaker.
What the two numbers mean
For a steel bolt, the class is written as X.Y. The first number is one hundredth of the nominal tensile strength in MPa; the second is ten times the ratio of yield strength to tensile strength.
| Class | Tensile Rm (MPa) | Yield Rp0.2 (MPa) | Yield / tensile |
|---|---|---|---|
| 4.8 | 400 | 320 | 0.8 |
| 8.8 | 800 | 640 | 0.8 |
| 10.9 | 1000 | 900 | 0.9 |
| 12.9 | 1220 | 1100 | 0.9 |
So 8.8 means 800 MPa tensile and a yield at 80% of that. Class 10.9 means 1000 MPa, yielding at 90%. The second number rising is what makes 10.9 and 12.9 behave differently in service: a higher yield-to-tensile ratio means less ductility before failure.
The failure mode you are actually choosing between
A fastener in a properly preloaded joint does not fail by being pulled apart. It fails one of two ways:
- Bolt fractures — the bolt is the weak link, which is the design intent. A broken bolt is visible and the joint does not come apart suddenly.
- Thread strips — the internal thread in the nut or tapped hole shears out. This is harder to inspect and often releases the joint with less warning.
Matching classes is how you control which one happens. The standard rule: the nut should be at least as strong as the bolt, so the bolt fails first. A class 8 nut under a class 12.9 bolt moves the failure into the nut thread, the opposite of what you want.
When a higher class is worse
Three cases where upgrading causes problems.
Hydrogen embrittlement. Class 10.9 and 12.9 fasteners that are electroplated (zinc, zinc-nickel) absorb hydrogen during plating. Under sustained tensile load, that hydrogen can cause sudden brittle fracture, sometimes days after assembly. Any plated fastener above class 10.8 must be baked within a defined window after plating. If you cannot control that in your supply chain, class 8.8 is the safer specification.
Reduced ductility. Class 12.9 has a yield-to-tensile ratio of 0.9, meaning it reaches yield at 90% of its ultimate strength. There is very little plastic deformation before fracture. In joints that see overload or shock, a 12.9 bolt can fail with almost no warning where an 8.8 would have stretched first and given visible evidence.
Stainless is not equivalent. A2-70 and A4-80 are stainless grades written the same way but they are not the stainless version of 8.8 and 10.9. The number is the minimum tensile strength divided by 100, so A2-70 is 700 MPa, below class 8.8. Stainless also has a different yield ratio. Never substitute stainless for a high class by matching the number.
What to specify in practice
- Start from the load, not the class. Work out the required preload, then pick the smallest class that reaches it with margin.
- Match the nut class to the bolt: class 8 nut for 8.8, class 10 for 10.9 and 12.9. See property classes for the full table including nut matching.
- For plated high-class fasteners, confirm the baking requirement in writing with the supplier.
- Where corrosion resistance drives the choice, go stainless and accept the lower strength rather than coating a high-class bolt you cannot bake.
If you already know the standard you need, the specification index lists every size with its dimensions, and each standard page carries its own selection notes — for example DIN 912 for socket head cap screws.