The numbers stamped on a bolt head are not decoration. They state the property class of the bolt — its strength — and checking them is one of the fastest incoming-inspection steps an importer can do.
Carbon steel: 4.8, 5.8, 8.8, 10.9, 12.9
For metric carbon steel bolts, the class is written as two numbers separated by a dot, for example 8.8. The first number is roughly one hundredth of the tensile strength in MPa, and the second number is the ratio of yield strength to tensile strength, times ten.
- 4.8 — tensile strength about 400 MPa. Common for light-duty, general-purpose fastening.
- 8.8 — tensile strength about 800 MPa. The workhorse grade for machinery, steel structures and automotive assemblies.
- 10.9 — tensile strength about 1000 MPa. Used where higher clamping force is needed.
- 12.9 — tensile strength about 1200 MPa. High-strength applications; usually supplied with a plain or black finish because electroplating can cause hydrogen embrittlement if the process is not controlled.
A higher class is not automatically better. The bolt, nut and joint must be designed together: a 10.9 bolt tightened into a weak base material will strip the thread or crush the joint instead of holding better.
Stainless steel: A2 and A4
Stainless bolts are marked differently, for example A2-70. The letter and first number describe the steel group: A2 is the common 304-type grade for general use, and A4 is the 316-type grade with better corrosion resistance, preferred for marine and chemical environments. The number after the dash (50, 70 or 80) states the strength level.
What to check when you order
- State the property class on the drawing or order, not just the size — “M10 bolt” is not a complete specification.
- Match the nut grade to the bolt grade; a lower-grade nut is the weak point of the joint.
- Ask for the material certificate with the shipment and compare it with the head markings on a sample from each carton.
- If your market uses inch sizes or ASTM grades, say so clearly — grade 5 and grade 8 inch bolts follow a different marking system (lines on the head, not numbers).
Send us your drawing or a photo of the bolt you are currently buying, and we will confirm the grade, material and finish item by item before quoting.