Grade 5 steel bolt. Three radial line marks. Theoretically, this bolt will feature a 120,000 psi minimum tensile strength.
The grade label specifies the minimum strength properties the fastener is intended to meet. Industrial fasteners are also marked with a label that identifies the maker of the fastener. Grade markings on domestic fasteners meet ASTM and SAE specs, while metric fasteners meet ISO and SAE specs.
Bolts whose heads are unmarked can be assumed to carry a grade of either 1, 2 or 4. These bolts will be made from low or medium carbon steel. At best, they may be cold-drawn, but certainly have not been tempered.
Contrary to popular opinion, there are more than only three grades of bolts (the most popular being grades 3, 5 and 8). There are about 17 grades (other grades may exist in aerospace or other niche industries).
A threaded fastener’s grade indicates its tensile strength and hardness. The higher the clamping load required, the higher the grade you’ll need. In automotive applications, although it’s difficult to make broad generalizations, you’ll need at least Grade 8 (or 12.9 in metric) for high-clamping load applications, such as connecting rods, cylinder heads and main caps, flywheel bolts, clutch cover bolts, etc. Never use any threaded fastener below a Grade 5 (or 8.8 metric) for any automotive application, regardless of the area of use. Always follow the vehicle or aftermarket component maker’s recommendation for fastener grade.
Grade 5 fasteners are made from medium-carbon steel that has been quenched and tempered. Minimum tensile strength is 120,000 psi for bolts up to one-inch diameter (tensile strength drops for diameters larger than 1 inch). Grade 5 bolt heads are marked with three radial lines. Rockwell hardness for bolts up to one-inch diameter is usually C25-34.
Grade 8 fasteners generally feature a minimum tensile strength of 150,000 psi and a Rockwell hardness of C38-39. Conventionally, a head marking of six radial lines indicate Grade 8.
Grade 9 fasteners (also called by trade names L9, PFC9, Tru-Torq and Bow Malloy) feature taller heads and 180,000 psi tensile strength and Rockwell hardness C38-42.