You can usually figure out the grade, or strength, of a bolt by looking at the markings on its head. English bolts (U.S.S. or S.A.E) are categorized by grade, from grade 2 to grade 8, where the grade represents inch strength. The higher the grade, the stronger the bolt. English bolts have radial lines on their head to identify the grade, or strength class. The number of lines on the bolt head is 2 less than the actual grade. So, for example, a grade 5 bolt will only have 3 radial lines on its head. A grade 8 bolt will have 6 radial lines on its head. Metric bolts are a bit more straight forward, usually having their strength class number stamped right on the bolt head. Some metric nuts also have the strength class number stamped on their face. The below diagram shows examples of English and Metric bolt and nut markings.
1. Grade 2 English Bolt
2. Grade 5 English Bolt
3. Grade 7 English Bolt
4. Grade 8 English Bolt
5. Class 9 Metric Nut
6. Class 4.6 Metric Bolt
7. Class 4.8 Metric Bolt
8. Class 5.8 Metric Bolt
9. Class 8.8 Metric Bolt
10. Class 9.8 Metric Bolt
11. Class 10.8 Metric Bolt
|US (English) Bolts|
|Grade and Material||Nominal Size Range (inches)||Proof Load (psi)||Min. Yield Strength (psi)||Min. Tensile Strength (psi)|
|Grade 2 - Low or medium carbon steel||1/4" - 3/4"||55,000||57,000||74,000|
|3-4" - 1-1/2"||33,000||36,000||60,000|
|Grade 5 - Medium carbon steel, quenched and tempered||1/4" - 1"||85,000||92,000||120,000|
|1" - 1-1/2"||74,000||81,000||105,000|
|Grade 8 - Medium carbon alloy steel, quenched and tempered||1/4" - 1-1/2"||120,000||130,000||150,000|
|Grade A325 - Carbon or alloy steel with or without Boron||1/2" - 1-1/2"||85,000||92,000||120,000|
|18-8 Stainless - Steel alloy with 17-19% Chromium and 8-13% Nickel||Up to 1"||20,000 Minimum|
|Class and Material||Nominal Size Range (mm)||Proof Load (MPa)||Min. Yield Strength (MPa)||Min. Tensile Strength (MPa)|
|Class 8.8 - Medium carbon steel, quenched and tempered||Up to 16mm||580||640||800|
|Class 10.9 - Alloy steel, quenched and tempered||5-100mm||830||940||1040|
|Class 12.9 - Alloy steel, quenched and tempered||16-100mm||970||1100||1220|
|A-2 Stainless - Steel alloy with 17-19% Chromium and 8-13% Nickel||Up to 20mm||210 Minimum|
Proof Load is the axial tensile load that the product must withstand without any evidence of permanent set.
Yield Strength is the maximum load at which a material exhibits a specific permanent deformation.
Tensile Strength is the maximum load in tension (pulling apart) that a material can withstand before breaking or fracturing.
1MPa = 145 pounds/inch²