Figuring Out Fastener Grades

Working in the maintenance, repair and operations (MRO) world, you are probably familiar with fasteners and the role they play. While fasteners are one of the smallest items you may find in a shop, they play a big role. Fasteners help hold everything together, ensuring that structures remain stable and secure.

There are three components of fasteners: the washer, nut and bolt. But not all fasteners are the same and choosing the right one can be confusing.

Fasteners come in different grades. The most common grades are Grade 2, Grade 5, and Grade 8.

Grade 2 fasteners are the most basic type and are typically made from low or medium carbon steel. They are typically used in applications that require low tensile strength. Tensile strength refers to the amount of force required to pull a fastener apart. Grade 2 fasteners have a minimum tensile strength of 74,000 psi.

Grade 5 fasteners are the most commonly used and best used for a variety of different applications revolving around machinery applications and automotive. They are made from medium-grade carbon steel and have a minimum tensile strength of 120,000 psi. They are distinguished by three radial lines on top of the bolt head.

Like Grade 5 fasteners, Metric 8.8 bolts are similar in applications and tensile strength and are best used in medium-strength applications within the mechanical and processing industries. Metric 8.8 head markings are distinguished by “8.8” on the bolt head.

A step up from Grade 5 are Grade 8 fasteners. They are best to use in high-stress applications, such as in heavy machinery. Grade 8 fasteners have a max tensile strength of 150,000 psi. They are distinguished by six radial lines on the top of the bolt head.

Similar to Grade 8 in application and tensile strength are Metric 10.9 bolts, which are best used for metric applications that revolve around high-stress applications too. Metric 10.9 bolts have a minimum tensile strength of 150,800 psi and is distinguished by “10.9” on the bolt head.

In addition to these grades, there are also specialized grades for specific applications. For example, Kimball Midwest has a line of stainless steel fasteners. Stainless steel fasteners are best used for high-corrosive environments such as marine, agriculture and food production industries. There are two types of stainless steel fasteners: 316 and 18-8. They have a similar tensile strength of 75,000 psi, but the 316 fasteners have a higher tolerance to corrosion compared to the 18-8.

Since there are different grades, it’s important to know the difference between them to ensure maximum safety and reliability. The nut should always be compatible with the bolt it is to be used with. For example, a Grade 5 nut should be used with a Grade 5 bolt. If you use a Grade 8 bolt with a Grade 5 nut, your fastening joint is only as strong as its weakest component. Using a stronger nut or bolt is not always better, so it is best to match the strength of the original fastener whenever possible. 

If you are unsure what fasteners would work best for you, a Kimball Midwest representative can help. If you don’t already have one, we can help you