SAE J429 Mechanical Requirements for Externally Threaded Fasteners

SAE J429 is the inch-system strength standard for externally threaded fasteners — the counterpart to ISO 898-1 for UNC and UNF bolts, screws and studs. It grades the common product as Grade 2, Grade 5 or Grade 8, and it is the reason an inch bolt carries radial lines on its head: Grade 5 has three lines, Grade 8 has six, and Grade 2 normally carries no strength marking at all.

Grade 2 is low or medium carbon steel and is the general-purpose, non-critical choice. Grade 5 is quenched and tempered medium carbon steel, the automotive and machinery workhorse. Grade 8 is quenched and tempered alloy steel for the highest inch loads, and reaches roughly 1034 MPa tensile — close to metric class 10.9. Unlike the metric classes, the quoted figures change with diameter: Grade 2 drops to a lower set above 3/4 inch and Grade 5 above 1 inch, so the size has to be read along with the grade. Match the nut to the bolt: a weaker nut on a stronger bolt simply moves the failure into the nut thread.

This standard fixes mechanical properties and test methods rather than dimensions, so there is no size table here. The head shapes and thread sizes it is applied to come from a separate product standard; the strength data itself — tensile, yield, hardness and the matching nut class — is on the class pages linked above.

The grades are not numerically comparable with ISO property classes. Grade 8 and class 10.9 land at a similar tensile strength, but the grades, thread forms, tolerances and marking rules belong to two separate systems, and one joint should not mix them. Head markings are the field check — three radial lines is Grade 5, six is Grade 8, and a head with no lines is Grade 2 or ungraded — and quenched and tempered parts additionally carry the manufacturer's identification mark. Those quenched and tempered grades also carry a minimum tempering temperature, which is a process requirement rather than a cosmetic one.