What do the numbers tell us?
On steel bolts, markings such as “8.8”, “10.9”, “12.9” on the head indicate the strength grade. This is not a random code — it gives direct information about the bolt’s tensile strength and yield limit.
The first number relates to tensile strength, the second number to the ratio of yield strength to tensile strength. The larger the number, the higher the load the bolt carries.
Common grades
- 8.8: The majority of general engineering applications. Machine assembly, steel construction, general industry.
- 10.9: Places where high strength is required. Automotive chassis joints, heavy machinery, critical structural connections.
- 12.9: The highest standard grade. Machine tools, high-stress special applications.
Is a higher grade always better?
No. Higher-grade bolts are harder but also more brittle — under sudden impact loads or in environments with hydrogen embrittlement risk, they can fail unexpectedly. The application determines the grade; the “higher is better” approach is wrong.
The risk of a wrong choice
Using a low-grade bolt under a high load results in yielding and fracture. Conversely, choosing an unnecessarily high grade both increases cost and, in some cases, introduces brittleness risk. The correct grade is determined by calculating the load the assembly will carry and the operating conditions.
The situation with stainless bolts
Stainless steel bolts use a different classification such as A2-70, A4-80. Here the letters indicate the material group, the numbers indicate strength. Stainless bolts generally have lower strength than an 8.8 steel bolt — a balance is struck between corrosion resistance and strength.