Reach into a bin of bolts and you will find four different marking systems mixed together. The lines and numbers on the head are not decoration; each system encodes the grade and, with it, the strength. This guide covers the SAE radial-line system first because it is the one people misread most, then metric, stainless, and the structural ASTM grades.
SAE Radial Lines: Count Plus Two
On an SAE inch-series bolt the head carries radial lines, and the grade is the line count plus two:
- No lines is a Grade 2, a low-strength general bolt (tensile around 60 to 74 ksi).
- Three lines is a Grade 5, medium carbon steel, quenched and tempered (120 ksi tensile).
- Six lines is a Grade 8, alloy steel (150 ksi tensile).
A plain head with only a maker mark is a Grade 2, not an unknown. The lines do not show the grade number directly, which is why people guess wrong: a Grade 8 shows six lines, not eight.
Bolt Grade Marking Decoder
Decode the lines and numbers on a bolt head. Reads SAE inch grades by radial-line count (Grade 2, 5, 8), metric property classes (8.8, 10.9, 12.9), ASTM structural grades (A307, A325, A490 under F3125), and stainless classes (A2-70, A4-80) into proof load, yield, and tensile strength, with the cross-system equivalents and the field cautions the chart leaves out. Source-cited to SAE J429, ISO 898-1, ISO 3506-1, and ASTM F3125.
Metric Property Classes Are Math
Metric bolts stamp a property class like 8.8, 10.9, or 12.9. It looks like a decimal but it is two encoded numbers:
- The first number times 100 is the nominal tensile strength in MPa. So 8.8 is 800 MPa, 10.9 is 1000 MPa, 12.9 is 1200 MPa.
- The first number times the second times 10 is the yield strength. So 10.9 is 10 x 9 x 10 = 900 MPa yield. The second number is the yield-to-tensile ratio in tenths.
Class 8.8 lines up roughly with SAE Grade 5, and 10.9 with Grade 8.
Stainless: Corrosion Class, Then Strength
Stainless fasteners stamp two things, and the first is corrosion, not strength. A2 is the 304 family. A4 adds molybdenum and is the 316 family, which resists chlorides, so A4 is the marine and chemical-service choice. The number after the dash is the strength class: 70 is 700 MPa tensile, 80 is 800 MPa. So A4-80 is a 316-type fastener at 800 MPa.
Structural Bolts: A325 and A490
Structural connections use ASTM bolts stamped with the grade, now published under ASTM F3125. A325 is roughly the strength of a Grade 5; A490 is roughly a Grade 8. The difference from a hardware-store bolt is that structural bolts are installed to a controlled pretension and the connection is designed around the specific grade. Two cautions: never substitute a lower or unmarked grade into a structural joint, and never hot-dip galvanize an A490 (hydrogen embrittlement risk at that strength).