When an engineer or procurement buyer orders hex bolts, the size is only half of the specification. The other half is the property class - the two-number grade stamped on the bolt head. Choose a grade that is too low and the joint can yield, fatigue, or fail in service; choose one that is far too high and you pay for strength you never use.
In the metric system, bolt strength is defined by ISO 898-1 (also adopted as DIN EN ISO 898-1). The property class uses two figures. The first, multiplied by 100, gives the nominal tensile strength in MPa. The second gives the ratio of yield strength to tensile strength multiplied by 10. For a class 8.8 bolt, tensile strength is 800 MPa and yield strength is 800 x 0.8 = 640 MPa.
Class 4.8 is the everyday commercial grade. It offers 400 MPa tensile strength and 320 MPa yield, typically in low- or medium-carbon steel supplied in the as-rolled or stress-relieved condition. Use 4.8 hex bolts for clamps, brackets, fencing, furniture, enclosures, and general fabrication where loads are static and modest. Many 4.8 bolts carry no raised grade marks on the head.
Class 8.8 is the workhorse grade for machinery. After cold heading, the bolts are quenched and tempered to reach 800 MPa tensile and 640 MPa yield. Grade 8.8 bolts are widely used in automotive assemblies, agricultural equipment, conveyors, machine frames, flanged connections, and structural steelwork designed around metric fasteners.
Class 10.9 steps up to 1000 MPa tensile and 900 MPa yield, usually in alloy steel such as 42CrMo4 or boron steel. You will find 10.9 bolts in suspension components, hydraulic clamps, presses, heavy trucks, crane sections, and high-load friction-grip joints. They should always be used with hardened washers where the bearing surface needs protection.
Class 12.9 is the strongest standard metric grade: 1200 MPa tensile, 1080 MPa yield. Because of its hardness (typically 39-44 HRC), 12.9 is specified for extreme-duty, space-limited designs - motorsport, tooling, gearboxes, and heavy machinery - where a smaller bolt must carry a large preload. Note that very hard steel has less ductility, so correct preload and installation procedure matter even more.
How do you identify the grade on a delivered part? ISO 898-1 heads usually carry the property class numbers - "8.8", "10.9", or "12.9" - alongside the manufacturer's mark. Many manufacturers additionally add radial identification lines. If neither appears, the bolt must not be assumed to be high strength; ask the supplier for mill test certificates.
A bolt is only as strong as the nut it threads into. Standard practice pairs each bolt with a nut of matching or higher class: class 8 nuts for 8.8 bolts, class 10 for 10.9, class 12 for 12.9. Mixing a low-grade nut with a high-grade bolt moves the failure point into the thread, often producing stripped threads instead of a controlled bolt failure.
Two practical tips for buyers: first, plating does not change the base grade, but high-strength parts above 1000 MPa are sensitive to hydrogen embrittlement, so electroplating must include proper hydrogen bake-out, while hot-dip galvanizing is common on structural grades. Second, preload controls performance - use a calibrated torque wrench and the correct torque value for the grade, size, and lubrication.
As a hex bolt manufacturer, Matrix Fasten supplies metric hex head bolts in classes 4.8 through 12.9, sizes M5 to M48, in zinc-plated, hot-dip galvanized, black oxide, and stainless steel finishes. Every production batch can be supplied with material and mechanical test reports, and custom lengths, head markings, and private-label packaging are available on request.
If you are comparing grade 8.8 bolts for an assembly line or need a container of mixed high-strength fasteners for a construction project, send us your drawings or specification list - our engineering team will confirm the right grade and finish within one working day.