When shopping for nuts and bolts, you may have noticed the fastener's head has markings engraved. These markings have a specific purpose that will help you understand the fastener more in-depth. The markings are an indication of the fastener's make, material, and dimension.
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The tool manufacturer engraves these markers as a helpful way for customers to be aware of what they are purchasing and using. Each manufacturer is required to engrave a unique marker that is specific to their brand. The brand marker is especially helpful as it allows you to trace the fastener back to the manufacturer if there is a defect. Watch the video below to learn more!
When it comes to nuts and bolts, there are many markings that you may come across. Common markers include letters, numbers, dashes, slashes, dots, and many more. Many nuts and bolts follow SAE or Metric standards ensuring strength and reliability.
The Society of Automotive Engineers created a system of grades to identify valuable information about the fastener. The grades of a fastener can represent what material it is made out of, its hardness range, and its strength characteristics. SAE grades use the imperial measurement system. The imperial measurement system uses inches.
The SAE J429 standard has specific requirements for bolts, screws, studs, sems, and U-bolts up to 1-1/2" in diameter. Bolts meeting SAE J429 standards have radial lines engraved on the bolt head.
Grade 2, 5, and 8 are the most common grades of fasteners according to the Society of Automotive Engineers standard. The higher the grade is, the stronger the material that makes up the fastener. The grade also represents the tensile strength, yield strength, and proof load of a fastener.
Does not have any radial line markings
Lowest SAE grade with the least strength
Comprised of low or medium carbon steel
Have three radial lines engraved
Medium level strength
Comprised of medium quenched and tempered carbon steel
Have six radial lines engraved
Highest SAE grade with the most strength
Comprised of medium quenched and tempered carbon alloy steel
Metric classes are set by the ISO (International Standards Organization). Metric markings combine two numbers separated by a dot. The number markings are engraved on the top or side of the bolt head. Common metric classes are 5.8, 8.8, 10.9, and 12.9. The higher the numbers are, the stronger the material of the fastener is.
The number that appears before the decimal, when multiplied by 100, will provide the approximate minimum tensile strength of the bolt. The number after the decimal, when multiplied by 10, will provide the approximate yield strength percentage in relation to the minimum tensile strength.
304 Stainless Steel Bolt Head Markings
Metric bolts made of 304 stainless steel are marked on the bolt head with A2-70. The A2 represents 304 stainless steel and 70 represents the tensile strength. The tensile strength equals 700 MPA general-purpose stainless steel. The number after the A2 will vary depending on the tensile strength of the bolt. 304 stainless steel has decent corrosion resistance.
316 Stainless Steel Bolt Head Markings
Metric bolts made of 316 stainless steel are marked on the head of the bolt with A4-70. The A4 represents 316 stainless steel and the 70 represents the tensile strength. The tensile strength equals 700 MPA marine grade stainless steel. The number after the A4 will vary depending on the tensile strength of the bolt.
The 316 stainless steel grade has high resistance to corrosion. The 316 stainless steel bolt is commonly used near salt water and a variety of other exterior applications.
Tensile Strength
Before using any fastener, being aware of its tensile strength, proof load, and yield strength is crucial so the fastener doesn't break or lose its elasticity during use.
Tensile strength is the amount of stress or load that the fastener can withstand by a material before it stretches and breaks. The tensile strength is tested by applying mechanical loads to the fastener. This amount of pressure determines its resilience. Understanding tensile strength is incredibly important when choosing hardware so it is clear if it is strong enough for the application.
Proof load is the limit of the elastic range of a bolt. If a bolt is tensioned beyond its specified proof load, it can't be used as it experiences plastic deformation. If it is tensioned within its specified proof load and has kept its original size and shape, it can be reused. Once the proof load is exceeded, it starts to yield and lose ductility.
Yield Strength
Lastly, yield strength is the maximum amount of stress a fastener can withstand before its shape is sufficiently deformed. Any deformation caused by stress greater than the yield strength results in the hardware being nonrecoverable for use.
(inches)
Mechanical Properties Proof Load(psi)
Min. Yield Strength(psi)
Min. Tensile Strength(psi)
(mm)
Mechanical Properties Proof Load(MPa)
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Min. Yield Strength(MPa)
Min. Tensile Strength(MPa)
640
800
16mm - 72mm 600 660 830940
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
Many screws have consistent markings that make it easy to distinguish the size of the screw. Conquest screws, for example, have the length in the decimal form for inches stamped on the head of the screw. Additionally, the Conquest concrete screws include a shield symbol stamped on the head of the screw that represents the Conquest brand.
The markings for concrete wedge anchors differ from screws. Conquest wedge anchors, for example, have a letter marked on the head of the anchor. The letter signifies the length of the fastener. Check out the spec sheet for the Conquest wedge anchors to see what the letters equate to in length sizes. Conquest wedge anchors also have a Conquest shield stamped on the clip at the bottom of the fastener.
When selecting bolts and nuts for any project, understanding grades is crucial. Bolt and nut grades are defined by both the material and the strength, which helps determine how much stress they can handle. Each grade has specific applications, and knowing the differences can help you choose the right fastener for your project.
Bolt and nut grades are established by various standards organizations and are used to indicate the material and strength of a fastener. Strength requirements are typically based on the material's hardness&#;stronger fasteners are made from harder materials. Grading systems often include proof load or tensile strength requirements, which verify that the material can withstand the listed forces.
There are several organizations that define bolt and nut grading standards, but three of the most widely recognized are SAE, ISO, and ASTM. These organizations create standards for different types of fasteners, including material requirements, strength ratings, and head markings. Below is an overview of how each standard works:
SAE is one of the most commonly referenced standards for inch-sized bolts and nuts in North America. SAE J429, for example, defines grades for bolts based on their strength, which is indicated by radial lines on the bolt head.
ISO 898-1 is the most commonly used international standard for metric bolts and nuts, also known as property classes. These fasteners are typically marked with numbers on their heads indicating their strength.
ASTM sets bolt and nut standards primarily for construction and industrial applications, with ASTM A307 being one of the most recognized bolt standards.
The table below provides a quick reference for comparing different bolt and nut grades across SAE, ISO, and ASTM standards, as well as their material types and common applications.
Grade/Class Strength (psi) Common Applications Standard Material SAE Grade 2 60,000-74,000 Non-critical applications (household repairs) SAE J429 Carbon Steel SAE Grade 5 105,000-120,000 Automotive, military, machinery SAE J429 Carbon or Alloy Steel SAE Grade 8 Up to 150,000 Heavy machinery, aerospace SAE J429 Carbon or Alloy Steel ISO Class 8.8 Comparable to Grade 5 Automotive, machinery ISO 898-1 Carbon or Alloy Steel ISO Class 10.9 Comparable to Grade 8 Heavy machinery, industrial ISO 898-1 Carbon or Alloy Steel ISO Class 12.9 Strongest metric Critical applications, engines ISO 898-1 Alloy Steel ASTM A307 Grade A 60,000 Non-critical construction ASTM A307 Carbon Steel ASTM A307 Grade B Up to 100,000 Piping, flanged joints ASTM A307 Carbon Steel Grade A Nuts Exceeds Grade 2 Light-duty, general use SAE Carbon Steel Grade B Nuts Similar to Grade 5 Medium-strength applications SAE Carbon Steel Grade G Nuts Similar to Grade 8 Structural, ASTM A325 bolts SAE Carbon Steel Stainless Steel A2-70 74,000 Corrosion-resistant applications ISO Stainless Steel Stainless Steel A4 180,000 Marine, chemical environments ISO Stainless Steel Titanium Grade 5 138,000 Aerospace, medical devices ISO TitaniumLike bolts, nuts are graded according to their material strength, and it&#;s important to match the grade of the nut with the grade of the bolt for optimal performance. Below are some common US nut grades:
In metric applications, nut strength is measured by property classes. Here are some common metric nut grades and their US equivalents:
Pairing nuts with the correct grade of bolt is essential to ensure the connection can handle the required load. Using a lower-grade nut with a higher-grade bolt could result in the nut stripping or failing, while using a stronger nut with a weaker bolt may not provide any additional benefits.
Grade 5 bolts are medium-strength fasteners with tensile strength between 105,000-120,000 psi, while Grade 8 bolts are high-strength with tensile strength up to 150,000 psi.
It&#;s generally recommended to match the grade of your nuts and bolts. Using a Grade 8 nut with a Grade 5 bolt may not offer any performance benefits and could result in failure if the bolt fails under load.
Metric Class 12.9 nuts are the strongest, followed by Grade 8 in US standards. These are used in critical applications like engine components and aerospace.
Most bolts have head markings that indicate their grade. For SAE bolts, radial lines on the bolt head indicate the grade. For ISO bolts, raised or stamped numbers indicate the property class.
Mixing metric and SAE fasteners is generally not recommended due to differences in size, thread pitch, and strength ratings. Always use matching nuts and bolts for the best performance.
At Monster Bolts, we offer a wide selection of high-quality fasteners. Whether you need high-strength Grade 8 bolts, 18-8 stainless steel bolts, or a variety of SAE and metric nuts, we have the right fasteners for your project.
For more information on fasteners, be sure to check out our other blog posts or reach out to our team of fastener experts.
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