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Bolt Grade Markings and Strength Chart

US Bolts

Head Marking Grade and
Material
Nominal Size Range
(inches)
Mechanical Properties
Proof Load
(psi)
Min. Yield Strength
(psi)
Min. Tensile Strength
(psi)
A307 Head Marking

307A

Low carbon steel
1/4" thru 4" N/A N/A 60,000
Grade 2 Head Marking
No Markings

Grade 2

Low or medium carbon steel
1/4" thru 3/4" 55,000 57,000 74,000
Over 3/4" thru 1-1/2" 33,000 36,000 60,000
Grade 5 Head Marking
3 Radial Lines

Grade 5

Medium carbon steel, quenched and tempered
1/4" thru 1" 85,000 92,000 120,000
Over 1" thru 1-1/2" 74,000 81,000 105,000
Grade 8 Head Marking
6 Radial Lines

Grade 8

Medium carbon alloy steel, quenched and tempered
1/4" thru 1-1/2" 120,000 130,000 150,000
A325 Head Marking

Grade A325

Carbon or alloy steel with or without boron
1/2" thru 1-1/2" 85,000 92,000 120,000
Stainless Markings Vary

18-8 & 316 Stainless

Steel alloy with chromium and nickel
All sizes thru 1" N/A 20,000 Min. 65,000 Typical 65,000 Min. 100,000 – 150,000 Typical
651 Silicon Bronze Head Marking

651 Silicon bronze

An alloy of mostly copper and tin with a small amount of silicon
1/4" thru 3/4" N/A 55,000 70,000
7/8" thru 1-1/2" N/A 40,000 55,000
Aluminum 2024 Head Marking

Aluminum 2024

Aluminum alloy with copper, magnesium and manganese; solution heat treated and age hardened
All sizes N/A 36,000 55,000

Metric Bolts

Head Marking Class and
Material
Nominal Size Range
(mm)
Mechanical Properties
Proof Load
(MPa)
Min. Yield Strength
(MPa)
Min. Tensile Strength
(MPa)
Class 8.8 Head Marking

Class 8.8

Medium carbon steel, quenched and tempered
All sizes below 16mm 580

640

800

16mm - 72mm 600 660 830
Class 10.9 Head Marking

Class 10.9

Alloy steel, quenched and tempered
5mm - 100mm 830

940

1040

Class 12.9 Head Marking

Class 12.9

Alloy steel, quenched and tempered
1.6mm - 100mm 970

1100

1220

Usually Stamped A-2 or A-4

A-2 & A-4 Stainless

Steel alloy with chromium and nickel
All sizes thru 20mm N/A

210 Min.
450 Typical

500 Min.
700 Typical

Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.

Yield Strength: The maximum load at which a material exhibits a specific permanent deformation.

Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set.

1MPa = 1N/mm2 = 145 pounds/inch2