If you’ve ever wondered what those screw and bolt head markings mean, or if there’s some overarching logic to them, you’re in the right place. In this article, we’ll give you an overview of the following:

Manufacturer Screw and Bolt Head Markings
Fastener Standard Screw and Bolt Head Markings
Examples of Screw and Bolt Head Markings & What They Mean

What Do Screw and Bolt Head Markings Mean?

Screw and bolt head markings identify the manufacturer of the fastener and the standard to which the fastener was made. Below, we’ll explore both of those concepts, then connect them together with a few examples.

Manufacturer Screw and Bolt Head Markings

The mandates that manufacturers must mark each fastener with their unique company symbol to identify that they made it — the only exception being fasteners that are too small to mark. Every company that has registered its specific screw and bolt head marking can be found in . Simply click the little PDF symbol next to the latest version, and you’ll be able to view the list for free.

Because each fastener is uniquely marked by its maker, the part can be easily traced back to its manufacturer should a problem occur. This traceability serves to instill a sense of accountability in the manufacturer and a sense of confidence in the end user.

As an example of a manufacturer screw and bolt head marking, the unique identifier for us at şÚÁĎÍř is the letter combination WG. Appearing on the head of a bolt or screw, it would look something like the image to the right.

Fastener Standard Screw and Bolt Head Markings

Over the years, different organizations have come together and attempted to bring some order to the fastener industry by releasing standards that provide specifications for certain types of fasteners. These specs can cover anything from material composition, to dimensional tolerances, to plating. For example, the American Society of Mechanical Engineers (ASME) has a document called ASME B1.1 that provides requirements for unified inch screw threads.

There are all sorts of standards for all sorts of fasteners — ISO, IATF, SAE, ASTM, ASME, etc. How important is a particular standard? It depends on how many people decide that it’s important. Some, like ASME B1.1 mentioned above, are universally accepted and used. Others are a bit more obscure, or are intended to service a limited segment of the industry.

Interested in learning about some other fastener standards? Check out our blogs on ISO standards or bolt grades.

Some fastener standards create classes or grades for fasteners by laying out material and physical requirements that a fastener must meet. A common example is SAE J429, which lays out requirements for many common fastener grades, such as Grade 2, Grade 5, and Grade 8 — which all begin to work as some kind of shorthand terminology for fasteners. When you know a fastener is Grade 5, for instance, you automatically know many details about it, including things like:

• Materials it’s made out of
• Its hardness range
• Its strength characteristics
• If it’s an inch or metric part

How does this all relate to screw and bolt head markings? Well, most fastener standards that introduce classes and grades have screw and bolt head marking requirements. The class and grade identifiers combine with the manufacturer’s head marking to comprise the jumble of stuff you’ll find on most fastener heads.

Examples of Screw and Bolt Head Markings & What They Mean

Now let’s put all this information together. We’ll do so by returning back to SAE J429, a standard established by The Society of Automotive Engineers (SAE). In a nutshell, this standard lays out mechanical and material requirements for inch bolts, screws, studs, sems, and U-bolts in sizes up to 1-½” in diameter.

But the main thing to focus on here is that the standard introduces a grading system based on numbers, where increasing numbers indicate increasing strength — so a Grade 5 has a higher tensile strength than a Grade 2. And those grades are shown on the head markings of a bolt. While SAE J429 standard designates many grades, we’ll focus on three of the most commonly used below.

SAE J429 Grade 2 Bolt Head Marking

Our first example is a bolt head marking that looks like this:

Wait a minute! There’s no grade marking on this bolt! You just reused the image from before! Well, that’s true — we did just reuse the image from before, but we did it because Grade 2 bolts don’t have a head marking requirement.

So if there’s no head marking requirement, how can you know when a bolt is a Grade 2 and when it’s something else? The answer is: we don’t know. A lack of a grade identifier will tell you that a bolt ISN’t one of the myriad of other classes and grades — but in order to be sure of exactly what it IS, you’ll need to verify the material.

Once we’ve established the bolt is a Grade 2, you now know two important things. You know that the bolt is an inch-series bolt, and you know what its minimum tensile strength is — well, once you know the diameter of the bolt, that is. Here are some of the minimum tensile strengths for Grade 2 bolts, based on diameter:

• Diameters of ÂĽ – ¾” = minimum tensile strength of 74 ksi
• Diameters of Âľ – 1 ½” = minimum tensile strength of 60 ksi

Bolts with these tensile strengths would be considered low-strength bolts. These strengths can generally be achieved without heat treating the parts.

SAE J429 Grade 5 Bolt Head Marking

Our second example actually has a grade head marking:

So, as you can see, this bolt head has şÚÁĎÍř’s manufacturer mark and three radial lines. These lines indicate that the bolt is a Grade 5. Here are some of the minimum tensile strengths for Grade 5 bolts, based on diameter:

• Diameters of ÂĽ – 1” = minimum tensile strength of 120 ksi
• Diameters of 1 – 1 ½” = minimum tensile strength of 105 ksi

Grade 5 bolts are considered to be medium-strength fasteners. They’re required to be heat-treated, quenched, and tempered in order to gain the desired strength.

SAE J429 Grade 8 Bolt Head Marking

Finally, our last example from SAE J429:

Here, we have a bolt head marking with the şÚÁĎÍř manufacturer identifier and six radial lines. These six radial lines indicate that the bolt is a Grade 8 bolt.

*Note: You may notice that in the images the WG identifier has been moved around a bit to accommodate the grade markings. This is a totally legitimate practice. Many times with smaller diameter fasteners, you’re trying to fit all the required markings on the head while keeping them legible — so you move things around to fit them as best as you can. For the SAE J429 standard, placement of the lines is important, so we move the WG around as necessary. For many other standards, you’re free to move all markings around as needed.

Back to Grade 8. Grade 8 bolts have a minimum tensile strength of 150 ksi for all diameters from ÂĽ – 1 ½”. They’re considered to be high–strength fasteners, and they need to be heat-treated, quenched, and tempered in order to reach the desired strength.

So there you have it — a quick overview of the key grades from SAE J429 and their head markings. As a quick summary:

• No marking – Grade 2
• Three radial lines – Grade 5
• Six radial lines – Grade 8

Learn More şÚÁĎÍř Screw & Bolt Head Markings with şÚÁĎÍř

Interested in learning a bit more about screw and bolt head markings? We talk about three additional standards that deal with inch-series fasteners in this blog. Or, we also talk about some metric head markings in this blog. Check them out.

And if you have any questions that these blogs can’t answer for you, feel free to reach out to us. With plenty of experience in the fastener industry, we’ll likely have an answer for you. Give us a call at (800) 656-2658 or contact us online.

The post What Do Screw and Bolt Head Markings Mean? appeared first on şÚÁĎÍř.

In the fastener world, you’ll often hear terms like proof load, yield strength, and tensile strength tossed around when referring to the strength of a given fastener. For those unfamiliar with the precise meanings of these terms, I thought I’d devote a blog post to help define them and their relation to one another.

Proof load, yield strength, and tensile strength are numbers set by a standard that a fastener must meet in order to qualify as a certain grade or property class. All three numbers are set as minimum (and occasionally maximum) values. For example, according to ASTM A354, in order for a ½-13 bolt to qualify as grade BD, it must have a minimum proof load of 17,050 pounds-force (lbf), a minimum yield strength of 18,500 lbf, and a minimum tensile strength of 21,300 lbf. Not all standards specify requirements for all three tests. Yield strength and proof load are similar tests, so yield strength requirements are often omitted in favor of proof load requirements, as in SAE J429.

Before I can talk about individual terms, I should talk a bit about the kind of fastener strength involved here. All three terms involve the load that a threaded fastener can hold when pulled perpendicularly from the head. See Figure 1.

In order to test this force, we use the tensile machine in our lab.

As you can sort of see, the fastener is fed into the slot in the middle. The machine then exerts a vertical force on the part. The machine measures the force as the part holds, distends, or breaks, depending on the test. To get an idea of how each test works, read on.

What Is Proof Load?

Proof load is an amount of force that a fastener must be able to withstand without permanently deforming. So, to use the example above, in order to pass the proof load test set by ASTM A354, a ½-13 bolt must be able to hold a load of at least 17,050 lbf for a minimum of ten seconds without permanently elongating. The length of the part is measured before and after the proof load test to ensure compliance.

What Is Yield Strength?

Yield strength is the load that is carried at the point where a fastener permanently deforms. When subjected to enough force, steel will begin to stretch. If the amount of force is low enough, the steel will elastically return to its original shape when the force is removed. At the yield point, the force becomes strong enough that the steel will stretch and not return to its original shape. This amount of force is the yield strength.

To test yield strength in our example, you would put our ½-13 bolt into the tensile machine, stretch the part until it distends, and calculate the force at the point of yield. In this case, the force would need to be a minimum of 18,500 lbf for the part to pass. The actual process of determining the force at the point of yield is rather engineer-y and involves graphs. If you would like to see it spelled out, 

What Is Tensile Strength?

A fastener’s tensile strength, or ultimate tensile strength, is the force at which the fastener fractures. To test tensile strength, we use a wedge tensile test, where a wedge is placed under the head of the fastener, and force is applied until the fastener breaks.

The wedge is used because it puts extra stress on the junction of the head and the body of the fastener. This ensures the absolute integrity of this junction. If the fastener breaks at a force greater than the minimum tensile requirement, the fastener has passed the tensile test. However, the break must not occur at the junction of the head and the body of the fastener. If the break does occur here, the fastener has failed tensile, regardless of the force at which the break occurred.

So to summarize, proof load is a load that can be held without permanent deformation. It is the lowest force of the three forces that we are discussing. Yield strength is the force exerted at which a fastener permanently deforms. Yield strength is a greater force than proof load. Finally, tensile strength is the force at which a fastener will break. It is the strongest of the three forces.

Before I sign off, I would like to point out that when a properly made fastener is subjected to a force greater than its tensile strength, it will break in a cross-section. In other words, the steel itself will give out across the diameter of the fastener before the threads shear. Threads are strong. Threads are cool. We talk about threads in more detail in our three-article series on threads. Part 1 provides a general introduction to threads. Part 2 talks about the difference between 2A and 3A threads. Finally, we wrap up with part 3, which discusses metric threads.

RELATED: Need a specialty bolt, screw, or stud? şÚÁĎÍř can help.

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In Part 1 of our Threads Series, we provided some terminology and explained some of the nomenclatures of Unified Inch series threads. Now, we’ll look at three particular classes of fit within that series, as we discuss the difference between 1A, 2A, and 3A threads.

What Are 1A, 2A, and 3A Fastener Threads?

The terms “1A,” “2A,” and “3A” refer to classes of fit for external Unified Inch series threads on screws, bolts, and studs. Internal threads, such as those found in nuts or tapped holes, have thread fit classes termed “1B,” “2B,” and “3B.” The A/B mating parts are designed to fit together to allow free-running assembly with no interference.

The term “thread fit” is defined as the measure of the looseness or tightness between mating threads when an externally threaded fastener is assembled into an internally threaded hole or nut. As you’ll see below, 3A/3B is a tighter fit than 2A/2B, and 2A/2B is a tighter fit than 1A/1B.

1A vs. 2A vs. 3A Threads

TL;DR — The key distinction between 1A, 2A and 3A threads lies in their thread fit characteristics and tolerance levels.

To understand the difference between these fastener threads, you must first understand thread fit, allowances, and tolerances. Basically, thread fits are developed using allowances and tolerances.Ěý

          An allowance is an intentional clearance between mating threads. Allowances are applied to external threads. The major, pitch, and minor diameter maximums are less than the basic size by the amount of the allowance.Ěý

          A tolerance is the difference between the maximum and minimum permitted limits for a given dimension. Tolerances are specified amounts by which dimensions are permitted to vary for manufacturing convenience.Ěý

And here’s the main difference between the three types of bolt, stud, or screw threads: 

• Class 1A threads have standard allowance and lots of tolerance (loosest fit)
• Class 2A threads have standard allowance and tolerance (medium fit)
• Class 3A threads have no allowance and close tolerance (tight fit)

The two diagrams below should nicely illustrate allowance and tolerance.

Here’s an Example

Let’s look at a specific example. This one compares 2A and 3A threads (we don’t manufacture parts with 1A threads at şÚÁĎÍř). The difference between class 2A and 3A external threads is shown below for 5/8-18 UNF parts.Ěý

Note that for class 2A, both the major and pitch diameter maximums are below their respective basic values by the 0.0014 inch allowance. The allowance is 30% of the class 2A tolerance.Ěý

For class 3A, the major and pitch diameter maximums are at the basic size. Also, the pitch diameter tolerance for class 3A is 0.0035 inches, which is smaller than the class 2A tolerance of 0.0047 inches.

A Side-by-Side Comparison

Now that we’ve examined an example of external threads, we are going to look at an entire mated thread system. Keeping the 5/8-18 example, let’s look at a side-by-side comparison of the complete clearance.Ěý

The complete clearance consists of the allowance (if any) for the external thread and tolerances for the external and internal mating threads. See Figure 6 below for a side-by-side comparison of 5/8-18 UNF 2A/2B and 3A/3B pitch diameter. The 3A/3B thread fit has no allowance and smaller tolerances than the class 2A/2B thread fit, resulting in a tighter fit.

Learn More şÚÁĎÍř Bolt and Screw Threads

We’ve provided some basic facts about Unified Inch series threads here. For more information, we recommend referring to the .Ěý

Our Threads Series continues in Part 3, where we provide a brief overview of metric series threads.

At şÚÁĎÍř, we produce small-quantity bolts, screws, and studs that are difficult to find everywhere else. View our capabilities, then contact us or request a quote to see how we can help you.

RELATED: Need a specialty bolt, screw, or stud? şÚÁĎÍř can help.

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