Bolt grades explained: 4.8, 8.8, 10.9 and 12.9 — how to choose
The two numbers on a bolt head mean something precise under ISO 898-1, and selecting the wrong one is both a safety question and, quite often, a cost question in the wrong direction.
What the numbers mean
For class 8.8, the first number multiplied by 100 gives the nominal tensile strength: 800 N/mm². The second number, multiplied by 10, gives the ratio of yield strength to tensile strength in percent: 80%, so 640 N/mm². Class 10.9 gives 1,000 N/mm² tensile and 90% yield. Class 12.9 gives 1,200 N/mm² and 90%.
| Class | Tensile strength | Yield strength (0.2%) | Typical use | Main caution |
|---|---|---|---|---|
| 4.8 | 400 N/mm² | 320 N/mm² | Covers, brackets, light assemblies | Low preload; unsuitable for dynamic loading |
| 8.8 | 800 N/mm² | 640 N/mm² | The default for most bolted joints | None significant |
| 10.9 | 1,000 N/mm² | 900 N/mm² | High-preload joints, engines, structures | Hydrogen embrittlement with electroplated coatings |
| 12.9 | 1,200 N/mm² | 1,080 N/mm² | Moulds, dies, robotics, high-stress fixtures | Most embrittlement-sensitive |
Why "use the strongest bolt" is usually wrong
Three reasons, in order of how often they cause problems:
- The joint, not the bolt, sets the limit. Bolting an aluminium casting with class 12.9 fasteners does not make the casting stronger. Over-tightening to exploit the bolt's capacity simply strips the female thread or crushes the mating part.
- Higher class is harder and more brittle. As strength rises, tolerance to impact and to stress concentration falls. In cold or dynamic service a very high class part can fail where a lower class would have deformed and survived.
- Embrittlement risk rises with strength. Electroplated high-strength parts absorb hydrogen during pickling and plating. Without a de-embrittlement bake, delayed fracture can appear days or weeks after assembly — with no warning and no visible defect at goods-in inspection.
Matching nuts and washers
A joint is only as strong as its weakest element. Property classes must be matched:
| Bolt class | Nut class (DIN 934 / ISO 4032) |
|---|---|
| 4.8 | Class 4 or 5 |
| 8.8 | Class 8 |
| 10.9 | Class 10 |
| 12.9 | Class 12 |
A higher nut class does no harm (the nut strips rather than the bolt, which is the preferred failure mode). A lower nut class makes the nut the weak link and can strip during tightening.
Coating choices by class
- Class 4.8 and 8.8: electroplated zinc is fine. No embrittlement concern at this strength level.
- Class 10.9 and 12.9: specify zinc-flake coatings (Dacromet, Geomet) or black oxide in preference to electroplating. If electroplating is unavoidable, require de-embrittlement baking within the specified window after plating and ask for the process record.
- Outdoor and marine: hot-dip galvanizing for structural items, A4 (316) stainless for smaller fasteners and for situations where hot-dip coating thickness would interfere with the joint.
What to put on the purchase order
- Product and standard (for example DIN 933 / ISO 4017)
- Size and length
- Property class
- Material
- Coating, including coating thickness or salt-spray requirement
- Marking requirements (class and maker mark are mandatory under ISO 898-1 above M5)
- Documentation: mill certificate, mechanical property report, coating thickness report
Everything on that list is confirmed back to you in writing before production starts — which is the point at which a specification error is still free to correct.