Sources and scope for this page: ISO 898-1:2013, covering the definition of property class (8.8, 10.9, 12.9). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
The number before the point is one tenth of the nominal tensile strength in MPa; the number after is the ratio of yield to tensile strength times ten. So a class 8.8 bolt has 800 MPa minimum tensile and yields at 80% of that — 640 MPa.
Higher classes are stronger but less ductile, and they are more sensitive to hydrogen embrittlement after plating, which is why 12.9 bolts are usually supplied with a protective coating rather than electroplated.
Common misuse
Substituting a higher property class without checking the joint. A stronger bolt changes the failure mode from bolt failure to thread stripping in the nut.