Literally SAE Grade 8, the quenched and tempered alloy steel bolt with 150 ksi minimum tensile, but on the floor it also means the heavy-duty choice for anything that matters. Stronger is not always better: Grade 8 has less ductility than Grade 5, gives less warning before it breaks and is more vulnerable to hydrogen embrittlement. Dropping it into a joint designed for Grade 5 adds no clamp load unless the torque goes up too.
Don't just throw Grade 8 at it, if the bolts keep breaking the joint's moving, fix the clamp load first.
Also heard as
- G8
- six-line bolt
Related terms
SAE grade
The inch bolt strength system, chiefly Grades 2, 5 and 8, identified by radial lines on the head.
Head marking
The grade symbols and maker's mark stamped on a bolt head, the main clue to its strength once it leaves the box.
Hydrogen embrittlement
Delayed brittle cracking of hard steel fasteners caused by hydrogen absorbed from plating, cleaning or corrosion.
Hardware-store grade
Dismissive term for unmarked, soft, zinc-plated bolts of unknown origin with no certified strength.