Bolt & Screw Head Markings and What They Mean
If you’ve ever spent any time looking at the heads of different fasteners, you’ve probably noticed that there are a wide variety of markings that may be stamped onto a fastener head. The markings can be numbers, letters, slashes, dots, or an assortment of other marks. If you’ve ever wondered what the markings mean, or if there is some over-arching logic to them, then you are in the right place.
Fastener head markings usually accomplish two things: they identify the manufacturer of the fastener and the standard to which the fastener was made. We’ll talk about manufacturer head markings first, followed by fastener standards and how they govern the industry. Finally, we’ll tackle some specific fastener markings, tie them to the standard, and talk about what it all means. I will focus on standards that deal with externally threaded fasteners, like bolts and screws, made from alloy steel, as these types of fasteners are Wilson-Garner’s specialty and what I know best.
Manufacturer Head Markings
The Fastener Quality Act mandates that manufacturers mark each fastener with a symbol to identify the company that made it. Each company has a unique identifier that they stamp onto the head of all the fasteners that they make. Of course, there are exceptions to this law, such as fasteners that are too small to mark, but don’t worry about it. Because each fastener is uniquely marked by its maker, the part can easily be traced back to its manufacturer should a problem occur. This traceability serves to instill a sense of confidence in the user and a sense of accountability in the manufacturer. Every company that has registered its symbol with the government can be found in this document. Simply click the little PDF symbol next to the latest revision, and you’ll be able to view the document for free.
As an example of a manufacturer head marking, the unique identifier for Wilson-Garner is the letter combination WG. Appearing on the head of a bolt, it would look something like this:
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, ASME, the American Society of Mechanical Engineers, 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. How important is a particular standard? That depends on how many people decide that it’s important. Some, like ASME B1.1 mentioned above, are universally used. Others are more obscure or are intended to service a limited segment of the industry.
Sometimes standards create classes or grades of 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 grades of fasteners, such as Grade 2, Grade 5, and Grade 8. I’ll expand on this particular standard in more detail below. These classes or grades begin to work as a kind of shorthand for fasteners. When you know a fastener is a Grade 5, for example, you automatically know many details about it. You know what materials it can be made out of, its hardness range, its strength characteristics, if it’s an inch or metric part, and all sorts of other information.
For the purposes of this blog, we’ll focus simply on whether a part is inch or metric and on the part’s strength characteristics, namely, tensile strength. In order to help with identification, most standards that introduce classes and grades have marking requirements. These class and grade identifiers combine with the manufacturer’s head marks to comprise the jumble of stuff that you’ll find on most fastener heads.
Some Examples – SAE J429 Grade 2, Grade 5, and Grade 8
The Society of Automotive Engineers (SAE) has a standard called SAE J429 Mechanical And Material Requirements for Mechanical Fasteners. In a nutshell, this standard lays out mechanical and material requirements for inch bolts, screws, studs, sems, and U-bolts. It covers sizes up to 1-1/2” in diameter. The main thing to focus on here is that the standard introduces a designation system based on numbers where increasing numbers indicate increasing tensile strength. So a Grade 5 has a higher tensile strength than a Grade 2. The standard designates more grades than the three that I have chosen to focus on, and there are some decimal grades, such as Grade 5.2. But I’ve chosen the above three grades because they are, in my experience, the most commonly used.
So let’s look at our first example, a 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, I did just reuse the image from before, but I did it because Grade 2 fasteners do not have a marking requirement. So if there’s no marking requirement, how can I know when a fastener is a Grade 2 and when it’s something else? The answer is, I don’t know. A lack of a grade identifier will tell you that a fastener ISN’T one of a myriad of other classes and grades. But in order to be sure, you’ll need to verify the material.
Once we’ve established that a 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. For diameters of ¼ inch through ¾ inch, the minimum tensile strength is 74ksi (thousand pounds force per square inch). For diameters over ¾ inch through 1-1/2 inches, the minimum tensile strength is 60ksi. Fasteners with these tensile strengths would be considered to be low strength fasteners. These strengths can generally be achieved without heat-treating the parts.
Here’s our second example, and this time there will actually be a grade marking:
So as you can see, this bolt head has Wilson-Garner’s manufacturer mark and three radial lines. These lines indicate that the bolt is a Grade 5. Grade 5 fasteners have a minimum tensile strength of 120ksi for diameters ¼ inch through 1 inch and 105ksi for diameters over 1 inch through 1-1/2 inches. Grade 5 fasteners are considered to be medium strength fasteners. They are required to be heat-treated, quenched, and tempered in order to gain the desired strength.
Finally, our last example from SAE J429:
Here we have a bolt head with the Wilson-Garner manufacturer mark and six radial lines. These radial lines indicate that the bolt is a Grade 8 bolt.
A quick aside – You may notice that in the images the WG mark has been moved around a bit to accommodate the grade markings. This is a totally legitimate practice. Many times with smaller diameter fasteners you are trying to fit all the required markings on the head while remaining legible. So you move things around to fit them as best as you can. For the SAE J429 standard, the placement of the lines is important, so we move the WG around as necessary. For many other standards, you are free to move all the markings around as needed.
Back to Grade 8. Grade 8 fasteners have a minimum tensile strength of 150ksi for all diameters from ¼ inch through 1-1/2 inches. Grade 8 fasteners are considered to be high strength fasteners. They are required to be heat-treated, quenched, and tempered in order to reach the desired strength.
So there you have a quick overview of the major grades from SAE J429. As a quick summary: No marking = Grade 2. Three radial lines = Grade 5. Six radial lines = Grade 8.
Our next post will talk about three additional standards that deal with inch-series fasteners, ASTM A307, ASTM A354, and ASTM A193.