Misunderstood Fastener Terms & What They Really Mean

In the fastener world, terminology matters. A lot.

Using the wrong term in a spec sheet, quote, or conversation can lead to incorrect product selection, confusion between teams, or even performance issues in the field. At Earnest Machine, we talk about fasteners all day, and we’ve noticed a handful of terms that tend to cause confusion more often than others. So, let’s break them down.

Whether you’re training a new team member, double-checking an order, or brushing up on your own industry knowledge, here are several fastener terms that are commonly confused with each other and what they really mean.

Strength Level (Grade vs. Class)

The term strength level refers to how much stress a fastener can handle before deforming or breaking, typically measured by its tensile and yield strength. For inch-sized fasteners, the strength level is referred to as a grade, often specified by the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials International (ASTM International).

SAE grades include Grade 2, Grade 5, and Grade 8. ASTM grades, however, include a wider range of strength levels including A307 B, A325, A193 Grade B7, A490, A449, and A354 Grade BD.

ASTM graded fasteners are often used in the construction of buildings, warehouses, bridges, and cell towers due to their ability to carry heavier loads. SAE Grades can be found in a variety of general-purpose applications such as automotive repairs, machinery assembly, and equipment maintenance where standardized strength and reliability are essential, but the extreme load requirements of ASTM grades are not necessary.

In contrast, lock nuts typically follow a different grading system specified by the American Society of Mechanical Engineers (ASME), which uses letter designations like A, B, C, F, and G. These ASME grades correspond to specific SAE bolt grades. For example, an ASME Grade B or Grade F lock nut is used with a Grade 5 bolt, and an ASME Grade C or Grade G lock nut is used with a Grade 8 bolt. Understanding these grade pairings ensures the lock nut and the mating bolt have matching strength levels for safe, reliable fastening.

Alternately, for metric sized fasteners, the strength level is referred to as a property class. For bolts and screws, common property classes include Class 8.8, Class 10.9, and Class 12.9. For nuts and lock nuts, property classes include Class 4, Class 5, Class 6, Class 8, Class 9, Class 10, and Class 12.

All these strength levels might sound or even look similar to each other, but don’t confuse the numbers (or letters) between the grading systems. For example, Grade 8 (inch) and Class 8.8 (metric) are not equivalent. In fact, Class 8.8 is closer in strength to Grade 5 than Grade 8.

Tensile Strength vs. Yield Strength

As mentioned above, a fasteners strength level is measured by its tensile and yield strength.

Tensile strength describes the maximum stress a fastener can handle before breaking while the yield strength describes the point where the fastener starts to deform permanently. The yield strength is often discussed in relation to another term called the yield point, which is the exact moment on a stress-strain curve when that permanent deformation begins. So, if you apply enough force past a fastener’s yield strength, it might not fracture but it won’t return to its original shape. This can lead to preload loss or failure in the joint or fastened assembly.

For a more in-depth explanation of fastener elasticity, read our Fastener Concepts article titled Strength in Stretching: Explaining Elasticity in Steel Fasteners.

Coarse Thread vs. Fine Thread

While coarse and fine threads are common across our industry, it’s important to remember that they are not interchangeable.

Coarse threads have fewer threads per inch (TPI) and deeper threads. They’re more resistant to stripping, easier to assemble, and better suited for use with softer materials. Fine threads have more threads per inch and a shallower thread depth. They offer better tensile strength and are more resistant to loosening caused by vibration when properly tightened.

Both coarse and fine threads are used across a wide range of fasteners including bolts, screws, nuts, and lock nuts. Lastly, each thread type is suited for a variety of applications. The best choice depends on the material, the environment, and how the fastener will be used.

Hardness vs. Toughness

These are often confused—even though they’re very different material properties.

Hardness describes a material’s resistance to surface deformation, like scratching or denting. While toughness describes a material’s ability to absorb energy and resist cracking or breaking under impact. A hard material isn’t always tough. In fact, extremely hard fasteners may be brittle, making them prone to sudden breakage under dynamic loads or impact. Take heat-treated fasteners, for example. They may offer excellent wear resistance but low ductility. That’s not a flaw; it just means they’re not right for every job. So, don’t assume a harder fastener always means a better fastener. Many applications require a balance of hardness and toughness for long-term performance.

Hex Head Cap Screw vs. Hex Head Bolt

They look nearly identical—but there’s a technical difference.

A hex head cap screw is precision-made, with tighter tolerances and a machined bearing surface under the head, often referred to as a washer face. This surface resembles an integrated washer and helps distribute the clamping load more evenly. A hex head bolt is typically used with a nut and does not have a washer face or as tight of tolerances. It’s often used in less critical applications.

In many cases, using the terms interchangeably won’t cause problems. But when a precise fit and strong, reliable clamping are required, hex cap screws are the better choice because their exact dimensions and built-in washer face help keep parts aligned and securely fastened.

This distinction is especially important when reviewing spec sheets or purchase orders. If someone asks for a “hex head bolt,” they might actually need a hex head cap screw—or vice versa—so it’s a good idea to double-check this information before submitting a quote or fulfilling an order. 

Wrap-Up: Why Terminology Matters

It’s easy to assume that everyone is speaking the same fastener language—but even small misunderstandings can lead to sourcing errors, failed joints, or unnecessary costs. That’s why we believe in keeping communication clear, accurate, and helpful.  

At Earnest Machine, we’re here to help you get it right the first time. If you need help sourcing fasteners, understanding spec sheets, or building your inventory, let us know. Our team is ready to help and offer solutions that meet your needs. 

Give us a call today at 800-327-6378 or email us at [email protected].