Building Fasteners FAQ | What Do Fastener Markings Mean?

A typical structure uses a lot of building fasteners. And the designers and contractors working on the project use fastener markings to distinguish between each type and application. But what do those fastener markings mean? Basics of Markings Most bolt markings are found on the fastener head. They can be a combination of numbers and […]

A typical structure uses a lot of building fasteners. And the designers and contractors working on the project use fastener markings to distinguish between each type and application. But what do those fastener markings mean?

Basics of Markings

Most bolt markings are found on the fastener head. They can be a combination of numbers and letters or a pattern of lines. Nuts tend to have much smaller markings, although they are also made of numbers and letters or lined patterns.
There are two meanings to a fastener marking:

• Original manufacturer – each manufacture will distinguish their product using a marking.

• Strength of the fastener – this is used as a guideline and assurance that the fastener meets quality assurance standards.

You will also find that the fastener markings display the type of material used and a range of nominal sixes. For instance, three straight lines that extend diagonally from the center of the head denote ASTM A449 fastener (Type 1), but the nominal size of that fastener could be anywhere from 1/4" to 3 inches. All lengths that fall within that range will carry the same distinct marking.

Who Designates the Markings?

Building fasteners fall under the watch of the ASTM (or the American Society for Testing and Materials). This body handles fasteners used on structures, while the SAE (or Society of Automotive Engineers) looks after the head markings on fasteners used in machinery and automotive applications.

Examples

There are different marks for externally threaded and internally threaded fasteners (bolts and nuts, for example). There are also differing marks for metric fasteners and those measured in inches.

Markings on a flanged nut are found on the top of the flange, whereas those found on a hex nut are displayed on the top of the nut itself.

Metric fasteners hold a property class designation, while those measured in inches are stamped with an identification mark displaying the grade of fastener. Metric fasteners with an underlined class marking are made from lower carbon steel, differentiating them from those made with medium carbon steels that have no underlined markings.

Fastener markings are used on building fasteners to provide quick information to those working with them. It is a simple form of identification and, if you often work with fasteners, one worth knowing.