Guide to bolt grades and applications

Read time: 10 minutes | Published on: 21 June 2023

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This guide is designed to help you understand different bolt grades and head types. Which bolt grades are commonly used in construction, manufacturing, and other industries? Discover the characteristics and specifications of bolt grades, along with the benefits and considerations of various bolt heads. We’ll cover:

What is the tensile strength of bolts?

The tensile strength of a bolt represents its capacity to endure stress and pressure. In simpler words, it indicates the maximum force a bolt can bear without failing. High-tensile bolt grades are capable of withstanding more significant impacts, making them suitable for demanding tasks, such as supporting industrial machinery or securing heavy objects securely. This should not be confused with yield strength, which is the amount of tensile strength at which specific permanent deformation happens.

The tensile strength of bolts varies depending on their grade or material composition. High-tensile bolt grades refer to bolts designed to withstand heavy loads and extreme conditions. These bolts are typically made from alloy steels and undergo specific heat treatments and manufacturing processes to enhance their strength.

The different types of bolt heads

Bolt head styles reflect the bolt’s intended function while enabling the installation tool to grip the head. The different types of bolt heads include:

Hex – the most common bolt head
Domed – resists tampering
Bent – for non-linear configuration
Square – enables a secure grip with a wrench or pliers
Pentagon – specialised applications where security is needed
Flat – ideal when space is limited

For a closer look at bolt heads, check out our guide, What are the Different Types of Nuts and Bolts?

What are the different grades of bolts?

Bolt grades, which indicate the strength of your fastener, are determined by the standard the bolt adheres to. There are three primary standards. Understanding bolt grades is critical to choosing the right one. Typically, identification markings on bolt heads include the grade and the manufacturer’s mark. Bolt grade markings are indicated by raised dashes or numbers.

You’ll also need to consider nuts and washers. To learn more about all of these fasteners and view bolt grade charts, see our guide, What Are the Different Types of Nuts and Bolts?

Bolt grades and applications

How to read bolt grades: there are three primary standards used worldwide, along with their bolt grade markings. The markings will tell you at a glance bolt strengths and grades.

Again, we refer you to our guide, which gives visual examples of each of the standards: What Are the Different Types of Nuts and Bolts?

Bolt grades explained: SAE

The first standard is SAE, which stands for the Society of Automotive Engineers. SAE bolt head markings use a series of raised dashes, or radial lines, to communicate strength.

SAE bolt grades and uses

Typical applications of SAE bolt grades include:

Grade 2	Low- to medium-strength carbon steel bolts used for non-critical joints: Light construction for wood framing, brackets, or other non-load-bearing applications Automotive assemblies with lower strength requirements, such as interior trim, non-structural components, or securing non-critical parts Plumbing & HVAC installations for non-structural elements such as brackets, supports, or covers
Grade 5	Medium carbon steel bolts used when more strength is needed than offered by Grade 2: Machinery and equipment – ideal for fastening components that experience moderate to high loads and vibrations. Typically found in industrial machinery, agricultural equipment, construction equipment, & other heavy-duty applications Automotive applications such as engine components, suspension systems & chassis assembly
Grade 8	High-tensile strength bolts used when superior strength and reliability are needed: Machinery and equipment for securing critical components subjected to heavy loads, vibrations, & harsh operating conditions Construction & infrastructure for connecting steel beams, columns, and other load-bearing applications

Grade 2

Low- to medium-strength carbon steel bolts used for non-critical joints:

Light construction for wood framing, brackets, or other non-load-bearing applications
Automotive assemblies with lower strength requirements, such as interior trim, non-structural components, or securing non-critical parts
Plumbing & HVAC installations for non-structural elements such as brackets, supports, or covers

Grade 5

Medium carbon steel bolts used when more strength is needed than offered by Grade 2:

Machinery and equipment – ideal for fastening components that experience moderate to high loads and vibrations. Typically found in industrial machinery, agricultural equipment, construction equipment, & other heavy-duty applications
Automotive applications such as engine components, suspension systems & chassis assembly

Grade 8

High-tensile strength bolts used when superior strength and reliability are needed:

Machinery and equipment for securing critical components subjected to heavy loads, vibrations, & harsh operating conditions
Construction & infrastructure for connecting steel beams, columns, and other load-bearing applications

Bolt grades explained: ASTM

The American Society for Testing and Materials (ASTM) indicates bolt grades by the letter A and three numbers on the bolt head.

A quick word about ASTM A325 grade: ASTM A325 bolts have been withdrawn as a standard specification. It was a specification for structural bolts that were commonly used in heavy construction applications. However, the ASTM F3125/F3125M standard has replaced the previous ASTM A325 specification. The new standard incorporates several bolt types, including the high-strength structural bolt previously covered by ASTM A325.

The change was made to align with the International Organization for Standardization (ISO) standards and to create a harmonised global standard for structural bolts. The new standard, ASTM F3125/F3125M, includes several grades and types of bolts, including the high-strength structural bolt now designated as "A325."

Therefore, while the specific ASTM A325 standard has been withdrawn, the high-strength structural bolt that was covered by that standard is still available under the ASTM F3125/F3125M standard as the "A325" bolt type. The designation ASTM A325 is still used in common vernacular.

ASTM bolt grades and uses

It’s worth noting the difference between SAE and ASTM bolt grades: ASTM A325 bolts are more often specified by engineers for structural steel connections on heavy construction projects. SAE Grade 5 bolts are favoured in OEM applications. That doesn’t mean there isn’t any crossover.

Grade A307	Low- to medium-strength carbon steel bolts used for non-critical joints: Light construction for wood framing, brackets, or other non-load-bearing applications HVAC for securing ductwork, piping, and other non-critical connections Electrical enclosures for securing components and providing access for maintenance or installations
Grade A325	Ensures the integrity and safety of structural connections: Construction and Building for connecting structural steel members, including beams, columns, & trusses Machinery and equipment for fastening structural elements, equipment, and machinery subjected to significant loads, vibrations, & dynamic forces
Grade A354 BD	Covers the mechanical requirements for alloy steel bolts, studs, & other externally threaded fasteners: Construction & infrastructure for connecting steel beams, columns, and other load-bearing applications. Structural bolting applications also include bridges, buildings, and other construction projects where high strength and reliability are required.

Grade A307

Low- to medium-strength carbon steel bolts used for non-critical joints:

Light construction for wood framing, brackets, or other non-load-bearing applications
HVAC for securing ductwork, piping, and other non-critical connections
Electrical enclosures for securing components and providing access for maintenance or installations

Grade A325

Ensures the integrity and safety of structural connections:

Construction and Building for connecting structural steel members, including beams, columns, & trusses
Machinery and equipment for fastening structural elements, equipment, and machinery subjected to significant loads, vibrations, & dynamic forces

Grade A354 BD

Covers the mechanical requirements for alloy steel bolts, studs, & other externally threaded fasteners:

Construction & infrastructure for connecting steel beams, columns, and other load-bearing applications. Structural bolting applications also include bridges, buildings, and other construction projects where high strength and reliability are required.

Bolt grades explained: Metric

Metric bolt grades are known as “property class,” and are set by the International Standards Organization (ISO). This system uses two numbers separated by a dot expressed in raised or depressed numbers either on the top or side of the bolt head. The higher the ISO numbers, the stronger the bolt.

Stainless steel bolt grades refer to the grade of stainless steel to make bolts. Stainless steel bolt strength grades are typically made of 304, 316 or 410 stainless steel. They’re bolt heads are marked as either A2 or A4.

Metric bolt grades and uses

Grade 8.8	Commonly used high-tensile bolt grade include the following applications: Automotive – engine components, chassis assembly, suspension systems, & critical structural connections Machinery & equipment for securing critical components subjected to high loads, vibrations, & harsh operating conditions Construction for structural engineering uses, such as bridges
Grade 10.9	Even stronger than grade 8.8: Heavy machinery, such as construction & mining equipment, agricultural machinery, and other heavy-duty applications Construction – structural steel connections that require high strength, such as bridges Automotive and racing, such as engine assemblies, suspension systems, & chassis components
Grade 12.9	This high-tensile bolt grade can handle all of the above, plus: Aviation for secure fastening for components in aircraft structures, engines, landing gear, & other high-stress areas Heavy machinery & equipment, construction machinery, mining equipment, offshore drilling rigs, & other heavy-duty applications Oil & gas for critical applications such as offshore platforms, pipelines, pressure vessels, & refineries
Grade A2	304 stainless steel bolts are used for: Food processing equipment for excellent corrosion resistance and hygiene properties. Used for securing equipment, conveyors, and components that come into contact with food or beverages
Grade A4	316 stainless steel bolts are typically used in: Marine environments due to superior corrosion resistance, particularly in saltwater environments. Used in boat building, dock construction, & other marine structures exposed to harsh conditions

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If you’re not quite sure which solution will work best for your application, our experts are always happy to advise you.

You can also read our guides, How to Prevent Loose Bolts and How to Measure Bolt Size.

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Questions?

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