validation of the strength of the bolt. However, low-cost fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar applications. Inch defines the bolt grades for inch-system sized bolts and screws. It defines them by grade, which ranges from 0 to 8, with 8 being the strongest. Higher grades do not exist within the specification. SAE grades 5 and 8 are the most common. []Head markings and properties for inch-system hex-head cap screws Head marking Grade, material, and condition Nominal size range (in) Proof strength (ksi/MPa) Yield strength, min. (ksi/MPa) Tensile strength, min. (ksi/MPa) Core hardness () SAE Grade 0 Strength and hardness is not specified SAE grade 1 ASTM A307 Low carbon steel 1⁄4–1-1⁄2 33/230 60/410 B70–100 ASTM A307 - Grade B Low or medium carbon steel 1⁄4–4 60/410 minimum 100/690 maximum B69–95 SAE grade 2 Low or medium carbon steel 1⁄4–3⁄4 55/380 57/390 74/510 B80–100 Greater than 3⁄4 33/230 36/250 60/410 B70–100 SAE grade 4 Medium carbon steel; cold worked 1⁄4–1-1⁄2 100/690 115/790 SAE grade 3 Medium carbon steel; cold worked 1⁄4–1 85/590 100/690 B70–100 SAE grade 5 Medium carbon steel; quench and tempered 1⁄4–1 (inc.) 85/590 92/630 120/830 C25–34 1–1-1⁄2 74/510 81/560 105/720 C19–30 ASTM A449 - Type 1 Medium carbon steel; quench and tempered 1–1-1⁄2 (inc.) 74/510 105/720 C19–30 1-1⁄2–3 55/380 90/620 183–235 SAE grade 5.1 Low or medium carbon steel; quench and tempered No. 6–1⁄2 85/590 120/830 C25–40
SAE grade 5.2 Low carbon martensitic steel; quench and tempered 1⁄4–1 85/590 120/830 C26–36 ASTM A449 - Type 2 Low carbon martensitic steel; quench and tempered C25–34 or - Type 1 Medium carbon steel; quench and tempered 1⁄2–1 (inc.) 85/590 92/630 120/830 C24–35 1–1-1⁄2 74/510 82/570 105/720 C19–31 ASTM A325 - Type 3 Atmospheric corrosion resistant steel; quench and tempered 1⁄2–1 85/590 92/630 120/830 C24–35 1–1-1⁄2 74/510 82/570 105/720 C19–31 - Grade BC Medium carbon alloy steel; quench and tempered 1⁄4–2-1⁄2 (inc.) 105/720 109/750 125/860 C26–36 2-1⁄2–4 95/660 99/680 115/790 C22–33 SAE grade 7 Medium carbon alloy steel; quench and tempered 1⁄4–1-1⁄2 105/720 115/790 133/920 SAE grade 8 Medium carbon alloy steel; quench and tempered 1⁄4–1-1⁄2 120/830 130/900 150/1,000 C32–38 ASTM A354 - Grade BD 1⁄4–2-1⁄2 (inc.) 120/830 130/900 150/1,000 C33–39 2-1⁄2–4 105/720 115/790 140/970 C31–39
Head marking Grade, material, and condition Nominal size range (mm) Proof strength (MPa/ksi) Yield strength, min. (MPa/ksi) Tensile strength, min. (MPa/ksi) Core hardness () Class 3.6 1.6–36 180/26 190/28 330/48 B52–95 Class 4.6 Low or medium carbon steel 5–100 225/32.6 240/35 400/58 B67–95 Class 4.8 Low or medium carbon steel; fully or partially annealed 1.6–16 310/45 340/49 420/61 B71–95 Class 5.8 Low or medium carbon steel; cold worked 5–24 380/55 420/61 520/75 B82–95 Class 8.8 Medium carbon steel; quench and tempered Under 16 (inc.) 580/84 640/93 800/120 17–72 600/87 660/96 830/120 C23–34 Class 8.8 low carbon Low carbon boron steel; quench and tempered Class 8.8.3 Atmospheric corrosion resistant steel; quench and tempered - Type 1 Medium carbon steel; quench and tempered 12–36 ASTM A325M - Type 3 Atmospheric corrosion resistant steel; quench and tempered