Metric bolts carry a property class stamped on the head — 8.8, 10.9, 12.9. The marking is defined by ISO 898-1, and it is more informative than most buyers realize: both numbers are engineering data, not a marketing scale.
How to read the numbers
The first number is nominal tensile strength in hundreds of MPa. The second is the ratio of yield to tensile strength, times ten:
- 8.8 — 800 MPa tensile, 80% yield ratio → 640 MPa yield
- 10.9 — 1000 MPa tensile, 90% yield ratio → 900 MPa yield
- 12.9 — 1200 MPa tensile, 90% yield ratio → 1080 MPa yield
Yield matters more than ultimate strength in most joints: a bolt that has yielded has lost its preload, and the joint is effectively loose even if nothing has broken.
Mapping to inch-series grades
Buyers mixing metric and inch specifications often need the rough equivalents:
- 8.8 ≈ SAE J429 Grade 5 (medium carbon steel, quenched and tempered) — general machinery and automotive.
- 10.9 ≈ SAE J429 Grade 8 (medium carbon alloy steel, quenched and tempered) — high-load structural and drivetrain work.
- 12.9 has no direct SAE bolt equivalent; it is the domain of socket head cap screws per ASTM A574, typical in 35CrMo / 40CrMo alloy steel.
When higher is not better
- Brittleness rises with strength. A 12.9 screw has less tolerance for bending, misalignment and shock loads than an 8.8. In joints with prying or uneven bearing, a lower grade in a larger size is often the safer design.
- Hydrogen embrittlement. Electroplating (zinc, yellow zinc) introduces hydrogen that can crack property class 10.9 and 12.9 parts after baking is skipped or insufficient. For high classes, specify mechanical zinc, black oxide, or zinc-flake coatings such as Dacromet, and ask for the baking record when plating is unavoidable.
- Threads and nuts must match. A 12.9 bolt with a commercial-grade nut fails at the nut. Pair with class 12 nuts, or A194 2H for B7-style assemblies.
Buying checklist
State the property class explicitly — "high strength" is not a specification. Add the standard (ISO 898-1, ASTM A574, ASME B18.3), material (e.g. 35CrMo), finish, and whether you need material certificates. Our socket head cap screws cover Grade 12.9 in alloy steel, and alloy steel fasteners group the high-tensile range across product types.
Not sure whether your joint needs 10.9 or 12.9? Send the application details — over-specifying costs money, under-specifying costs joints.
