Two digits separated by a dot on a bolt head are the densest notation in fastening. Tensile strength, yield strength and permissible tightening torque all follow from them. Here is how to read it, and where people get it wrong.
What the two digits mean
The marking is defined by ISO 898-1 (in the Russian system — GOST R ISO 898-1, formerly GOST 1759.4). Neither digit is a part number or a series — both are working values:
- First digit × 100 — ultimate tensile strength σu, MPa.
- First × second × 10 — yield strength σy, MPa.
Class 8.8: σu = 8 × 100 = 800 MPa, σy = 800 × 0.8 = 640 MPa. The second digit is the ratio of yield to ultimate strength — the margin between the onset of permanent deformation and failure.
| Class | σu, MPa | σy, MPa | Typical use |
|---|---|---|---|
| 4.6 | 400 | 240 | Non-critical joints, general assembly |
| 4.8 | 400 | 320 | Sheet structures, housings |
| 5.8 | 500 | 400 | General machinery, moderate loading |
| 6.8 | 600 | 480 | Transitional class, rarely specified |
| 8.8 | 800 | 640 | The workhorse: steel structures, flanges, machinery |
| 10.9 | 1000 | 900 | Critical joints, lifting equipment, heavy machinery |
| 12.9 | 1200 | 1080 | Hydraulics, dies, highly loaded assemblies |
For class 8.8 the standard splits two diameter ranges: up to and including M16, σu = 800 MPa; above M16 — 830 MPa. In practice the “same class” in a larger diameter behaves differently, and the range must be taken into account when calculating tightening torque.
Nuts are marked differently
A nut carries one digit, and it does not mean the same thing: the class number multiplied by 100 gives the nut’s proof stress in MPa. A class 8 nut holds 800 MPa, a class 10 nut — 1000 MPa.
The matching rule is simple: the number on the nut must be no lower than the first digit on the bolt. A class 8 nut or higher for an 8.8 bolt; a class 10 or 12 nut for a 10.9 bolt. If the nut is weaker, the joint fails by thread stripping in the nut rather than through the shank — and it fails below the design load.
Stainless fasteners run on their own scale
Austenitic stainless steels use a different marking: a letter with a group number and a figure after a hyphen. A2-70: A2 is a 304-type steel, 70 means an ultimate strength of 700 MPa. A4-80: A4 is a molybdenum-bearing steel (AISI 316), 800 MPa.
The point regularly missed: A4-80 is roughly comparable to class 8.8 in strength but does not substitute for it in calculations — stainless has different ductility, different behaviour under tightening and a marked tendency to thread galling. Conversely, class 8.8 says nothing at all about corrosion resistance.
Five mistakes that strip fasteners
1. A nut weaker than the bolt. The most common one. A 10.9 bolt with a class 6 nut will fail through the nut thread well before the design load. The nut marking deserves the same attention as the bolt marking.
2. Electroplating high-tensile fasteners. For class 10.9 and above, electrolytic coating carries a hydrogen embrittlement risk: hydrogen absorbed during deposition causes delayed failure hours or days after tightening. ISO 4042 requires a de-embrittlement bake after coating; for 12.9, sherardising or a zinc-flake coating is the safer route.
3. Confusing class with corrosion resistance. “Give me something stronger, stainless 8.8” is a combination that does not exist. Strength class and corrosion resistance are independent characteristics with separate marking systems.
4. Reading the nut digit as a bolt class. A nut marked “8” and a bolt marked “8.8” are different quantities expressed under different rules. The error puts a nut one class below the requirement into the specification.
5. Treating the marking as proof. The stamp on the head is applied by the manufacturer. Without a certificate and a test report it is a claim, not evidence — and on critical projects, particularly under 44-FZ and 223-FZ, acceptance will require the document.
What to check before ordering a batch
- Bolt strength class and nut class — matched to each other.
- Diameter relative to the M16 threshold, if the class is 8.8.
- Coating — compatible with the class (see mistake 2).
- Certificate of conformity and mechanical test report available.
- For import substitution — GOST ↔ DIN/ISO equivalence by class, not by geometry alone.
If your specification needs selection with a tightening-torque calculation, or a GOST-to-DIN equivalence check — send us the specification and we will return the calculation within one business day.