ISO 898-1 — Metric Bolt Classes 8.8, 10.9 & 12.9
The metric strength system. The number stamped on the head tells you how strong the bolt is — and it’s easier to read than it looks.
ISO 898-1 sets the property classes for carbon- and alloy-steel metric bolts, screws and studs. The class is stamped on the head — 8.8, 10.9 or 12.9 are the ones you’ll meet on machinery, vehicles and equipment.
Covers
Metric bolts, screws & studs
Identify by
Class number on the head (8.8, 10.9, 12.9)
Typical use
Machinery, vehicles, imported equipment
How to read the number
The two numbers are a code. The first number × 100 is the minimum tensile strength in megapascals — 8 means 800 MPa. The number after the dot is the yield as a share of tensile — .8 means yield is 80% of tensile (640 MPa). So a 10.9 bolt is 1,000 MPa class with yield at 90% of it.
Strength by class
| Class | Proof | Yield | Tensile | Hardness |
|---|---|---|---|---|
| 8.8 (up to M16) | 580 | 640 | 800 | HRC 22–32 |
| 8.8 (over M16) | 600 | 660 | 830 | HRC 23–34 |
| 10.9 | 830 | 940 | 1,040 | HRC 32–39 |
| 12.9 | 970 | 1,100 | 1,220 | HRC 39–44 |
Inch equivalents (roughly)
8.8 sits close to SAE Grade 5, 10.9 close to Grade 8, and 12.9 is the class most metric socket head cap screws are made to. They’re near neighbors, not identical — when a drawing calls for a metric class, use that class.
Matching nuts
Pair the bolt with a nut of the matching class from the companion standard, ISO 898-2: a class 8 nut on an 8.8 bolt, class 10 on 10.9, class 12 on 12.9. A weaker nut can strip before the bolt reaches its strength.
Plating high-strength metric bolts. 10.9 and especially 12.9 parts can be weakened by hydrogen from electroplating. Buy them already finished by the maker rather than plating them afterward.
We stock metric 8.8, 10.9 & 12.9
Metric hex cap screws, socket head cap screws, flange bolts and nuts by class. Give us the size, length and class.