Scope
ISO 898-1 is the international specification for mechanical properties of carbon and alloy steel bolts, screws, and studs in metric sizes. ASTM F568M is the US counterpart, harmonized with ISO 898-1 so that the property classes are identical and fasteners meeting either spec are interchangeable.
Metric bolts are designated by diameter × pitch (M10 × 1.5) plus property class (8.8, 10.9, etc.). The property class system replaces the SAE radial-line system used on inch bolts.
The property class system — how to read it
A property class designation like "10.9" encodes both tensile and yield strength in a simple format:
- First number × 100 = minimum tensile strength in MPa
- First number × second number × 10 = minimum yield strength in MPa
So property class 10.9 means:
- Min. tensile = 10 × 100 = 1000 MPa (about 145 ksi)
- Min. yield = 10 × 9 × 10 = 900 MPa (about 130 ksi)
- Yield is 90% of tensile
And property class 8.8 means:
- Min. tensile = 8 × 100 = 800 MPa (about 116 ksi)
- Min. yield = 8 × 8 × 10 = 640 MPa (about 93 ksi)
- Yield is 80% of tensile
Once you know the rule, you can read any metric bolt class at a glance.
Common property classes
| Class | Min. Tensile | Min. Yield | Material | Rough SAE equivalent |
|---|---|---|---|---|
| 4.6 | 400 MPa (58 ksi) | 240 MPa (35 ksi) | Low / medium carbon | SAE Grade 1–2 |
| 4.8 | 400 MPa | 320 MPa (46 ksi) | Low / medium carbon, cold-worked | — |
| 5.8 | 500 MPa (73 ksi) | 400 MPa (58 ksi) | Low / medium carbon, cold-worked | — |
| 8.8 | 800 MPa (116 ksi) | 640 MPa (93 ksi) | Medium carbon, Q&T | SAE Grade 5 |
| 9.8 | 900 MPa (131 ksi) | 720 MPa (105 ksi) | Medium carbon, Q&T | — |
| 10.9 | 1000 MPa (145 ksi) | 900 MPa (131 ksi) | Medium carbon alloy, Q&T | SAE Grade 8 |
| 12.9 | 1200 MPa (174 ksi) | 1080 MPa (157 ksi) | Alloy steel, Q&T | Stronger than Grade 8; ≈ A574 SHCS class |
Class 8.8 and the size quirk
For class 8.8, the ISO standard previously specified slightly lower strength for bolts ≤M16 vs larger sizes. Current ISO 898-1 unifies property requirements across sizes, but older documentation and legacy drawings may reference this distinction. Always verify the edition of ISO 898-1 called out on the drawing.
Marking
Metric bolts are marked with the property class on the head:
- 8.8 — stamped "8.8"
- 10.9 — stamped "10.9"
- 12.9 — stamped "12.9"
- Some manufacturers use an alternative dot / clock-mark system, but numeric marking is standard
- Manufacturer's identification symbol accompanies the class
Unlike SAE inch bolts, there is no radial-line system on metric bolts. The numeric mark is the only identifier.
Recommended nuts
Metric nuts are property-classed by a single number denoting proof load strength in MPa / 100:
| Bolt class | Nut class | Proof load stress |
|---|---|---|
| 4.6, 4.8, 5.8 | 05 | 500 MPa |
| 8.8 | 8 | 800 MPa |
| 10.9 | 10 | 1040 MPa |
| 12.9 | 12 | 1150 MPa |
Match the nut class to the bolt class. Class 8 nut with 8.8 bolt; class 10 nut with 10.9 bolt; class 12 nut with 12.9 bolt. Nut specs are covered by ISO 898-2 and ASTM F836M.
Applications
- Automotive assembly (European and Asian brands especially)
- Global OEM equipment (anything destined for European or Asian markets)
- European-spec construction and industrial equipment
- Machinery built to DIN, ISO, or JIS standards
- Metric-based aerospace and defense programs
- Import substitution — replacing OEM metric fasteners with domestic stock
Metric threads and tolerance
Metric threads follow ISO 68-1 profile, with tolerance classes that differ from inch practice:
| Thread class | Application |
|---|---|
| 6g (external) / 6H (internal) | Standard commercial fit — most common |
| 4g6g (external) / 5H6H (internal) | Precision fit |
The "g" / "H" designations replace the 2A/2B or 3A/3B of inch threads. 6g/6H is the everyday metric fit and the default when no class is specified.
What to watch for
- Dimensional substitution. M12 is not 1/2" — it's about 0.472". Don't cross-substitute diameters without checking.
- Thread pitch. Metric bolts can be coarse or fine pitch (M10 × 1.5 coarse vs M10 × 1.25 fine). Confirm the callout.
- Property class ≠ SAE grade. 8.8 is "about the same" as Grade 5, but they are not interchangeable for certified applications. If a drawing calls for property class 10.9, substitute with Grade 8 only if the specifier approves.
- Low-strength classes. 4.6 bolts look identical to high-strength bolts but are weaker than SAE Grade 2. Check markings on any metric bolt from unknown supply.
Related specifications
- ISO 898-2 / ASTM F836M — Metric nuts (matching class)
- ISO 4014 / DIN 931 — Hex-head bolt dimensional standard (partial thread)
- ISO 4017 / DIN 933 — Hex-head bolt dimensional standard (full thread)
- ISO 4762 / DIN 912 — Socket-head cap screw dimensional standard
- ISO 68-1 — Metric thread profile
- F568M — US version of ISO 898-1
Documentation
California Fastener F568M / ISO 898-1 orders ship with mill certificates showing property class, heat number, chemistry, mechanical properties, and thread geometry. Global-spec compliance statements available for OEM export programs.