Sourcing guides
How to choose bolt grade: 4.8 vs 8.8 vs 10.9
Grade 4.8, 8.8 and 10.9 look almost identical on the shelf. The difference only shows up under load — and in the price. Here is how to pick the right one without over-paying.
What the number actually means
The grade is the minimum tensile strength in kgf/mm², written as two numbers: the first is 1/100 of the tensile strength, the second is 1/10 of the yield ratio. A 8.8 bolt therefore has 800 N/mm² tensile and yields at 80% of it.
| Grade | Tensile (N/mm²) | Typical use |
|---|---|---|
| 4.8 | 400 | Light structures, brackets, general assembly |
| 8.8 | 800 | Machinery frames, most structural bolting |
| 10.9 | 1000 | High preload, flanges, engine mounts |
| 12.9 | 1200 | Highly stressed joints, tooling |
Two things buyers get wrong
- Grade is not finish. A zinc-plated 8.8 and a plain 8.8 are the same bolt mechanically; the plating only changes corrosion life.
- Higher is not always better. Above 10.9 the bolt gets harder and more brittle — it tolerates less misalignment and can crack instead of stretching.
If you are unsure, tell us the joint, the load and the environment — we will come back with the grade and the finish that fits, not the most expensive one.
Comments (1)
Useful. Do you also supply 10.9 with hot-dip galvanising? We use them on harbour cranes.