A strength classification for inch steel bolts and screws. Grade 2 is low-carbon steel with no head marks, Grade 5 is quenched and tempered medium-carbon steel with 120 ksi minimum tensile up to 1 in diameter and three radial lines, and Grade 8 is quenched and tempered alloy steel with 150 ksi minimum tensile and six radial lines. Nuts are graded to match the bolt.
Three lines on the head, that's SAE Grade 5, fine for the bracket, but the spring hanger wants Grade 8.
Count the lines and add two: three lines is Grade 5, six lines is Grade 8.
Also heard as
- SAE J429 grade
Do not mix it up with
- Property class: Property class is the metric system, stamped as numbers like 8.8 and 10.9.
Related terms
Grade 8
Shop shorthand for the strongest common inch bolt, and loosely for anything overbuilt; six 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.
Property class
The metric strength grade of a bolt or nut, stamped on the head as a number such as 8.8, 10.9 or 12.9.
Hardware-store grade
Dismissive term for unmarked, soft, zinc-plated bolts of unknown origin with no certified strength.