What do the numbers stamped on a bolt head mean?
8.8, 10.9, A2-70 — the two marking systems, what strength they describe, and why a stainless bolt is not automatically an upgrade.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 3 min
Short answer
Numbers like 8.8, 10.9 and 12.9 are steel property classes: the first figure relates to tensile strength and the second to how close to that the bolt yields. Higher is stronger and more brittle. A2-70 and A4-80 are stainless: the letter and number give the grade, the last figure gives the strength class — and both are weaker than an 8.8 steel bolt.
The marks on a bolt head are not decoration or a maker's logo. They are a specification, and on anything that carries load — a vehicle, a machine, a structural connection, a trailer, a climbing frame — swapping a marked bolt for an unmarked one from a jar is a real reduction in the strength of the joint.
Step by step
- Read the steel property class as two figures.Marks like 4.6, 8.8, 10.9 and 12.9 are the international property class. The first number relates to the ultimate tensile strength and the second to the ratio of yield to tensile — how much of its strength you can use before it deforms permanently.
- Know where each class turns up.4.6 and 4.8 are ordinary mild steel bolts, fine for general fixing. 8.8 is the standard high-tensile bolt used in vehicles, machinery and structural connections. 10.9 and 12.9 are higher strength again and are found in engines, suspension and precision machinery.
- Understand the trade-off.Higher grades are stronger but less ductile: they stretch less before they break, and they are more sensitive to corrosion, notches and hydrogen embrittlement. Higher is not automatically better.
- Read the stainless marking differently.A2-70 or A4-80 means grade A2 or A4 austenitic stainless, at strength class 70 or 80. The letter tells you about corrosion resistance and the number about strength. They are not interchangeable with the steel classes.
- Do not treat stainless as an upgrade.A2-70 is roughly comparable with a mid-grade steel bolt and considerably weaker than 8.8. Choosing stainless for corrosion resistance can mean accepting a weaker fastener, which matters where the joint is highly loaded.
- Replace like with like on anything load-bearing.Vehicle suspension, brakes, towbars, trailer components, machinery, structural brackets. Match the class, the size and the pitch, and use a new nut where the original was a locking type.
- Treat unmarked bolts as low grade.Marking is required on graded fasteners. An unmarked bolt should be assumed to be at the bottom of the range, whatever it looks like.
- Note the nut has a class too.Nuts are graded — 8, 10, 12 — and a high-tensile bolt with a low-grade nut fails at the nut. Buy them as a pair for anything that matters.
Tips
- The marks are usually on the top face of a hex head, sometimes with the manufacturer's identifier alongside. On socket cap screws they are often on the top face around the drive.
- Galvanising a high-tensile bolt can cause hydrogen embrittlement if done badly, which is why high-grade fixings for outdoor use are usually specified as a coated or stainless product rather than dipped afterwards.
- For domestic fixing — brackets, furniture, gates — a 4.6 or 8.8 zinc-plated bolt of the right size is almost always the sensible choice, and corrosion resistance matters more than grade.
Common mistakes
- Replacing an 8.8 bolt with an unmarked one from a jar — You may be halving the strength of a joint that was specified for a reason. On a vehicle or a towing component that is a safety issue.
- Assuming stainless is stronger because it costs more — Corrosion resistance and strength are different properties. A4-80 is excellent in salt water and still weaker than a plain 10.9 steel bolt.