What Are SAE Bolts?

SAE is an engineering group that writes industry standards. Among those standards are the fractional inch bolt, nut and screw sizes that we all use at home or work.

The tensile strength, or resistance to breaking, of bolts can vary by grade or material composition. Most bolts have special markings or grades, usually on the head, which indicate their tensile strength.
Thread Pitch

Bolts and other fasteners are used to hold machines together, so it’s important that they work properly. One way to ensure that a bolt fits its intended fastening system is by matching its thread size.

A bolt’s thread size is determined by how many threads it has per inch. The number identifies how coarse or fine the threads are. For example, 1/4″ bolts can be labeled as “SAE standard coarse,” or as “SAE standard fine.”

The naming of thread sizes is based on the use of each bolt type. For instance, the SAE standard coarse thread is primarily found in automotive horsepower and aerospace industries. However, the SAE standard fine is commonly found in other types of machinery. This difference between thread sizes is one of the reasons why it’s essential to understand the various uses for different kinds of bolts before ordering them. Having the wrong kind of thread can result in loosening or tightening faster than necessary, and can even damage the machinery.
Grade

The SAE system has a number of grade standards for inch-sized bolts. Each grade represents a different level of strength, with higher numbers representing stronger material. These grades are based on the type of steel used to manufacture the bolt and how it is treated.

Bolt grades start with grade 2, which is identified by its lack of head markings and has a tensile strength around 64,000 pounds per square inch (psi). Grade 5 bolts, marked with three raised radial lines in the head, are made from tempered medium carbon steel and have a tensile strength of 105,000 psi or more.

The strongest commercial-grade bolts are grade 8. They are produced from quenched and tempered medium alloy carbon steel and have a tensile of 110,000 psi or more. These bolts are recognised by six lines pressed into the head of the bolt. They are used in key components such as key brackets and steering and suspension parts.
Head Marking

Many fasteners follow SAE or metric standards that ensure strength and reliability. These standards are accompanied by a numbering system and head markings that indicate valuable information about the fastener’s materials, hardness range, and strength characteristics.

The head markings are a series of lines that appear on the top of the bolt’s head. They can include a letter combination or a series of dashes. The head markings can also indicate who manufactured the bolt. This will vary from manufacturer to manufacturer.

The lines on a bolt’s head can also indicate its grade. For example, a bolt with three radial lines will be Grade 5. Bolts with six radial lines are Grade 8. This is the highest grade and is made of medium carbon steel that has been case-hardened. This is the kind of high-strength bolt that’s often used in industrial applications. This grade is also commonly used in automotive applications. These bolts are typically forged and hot-rolled.
Material

Bolts can be made from different types of metal, with the most common being steel. The strength of the bolt depends on its grade, with higher grades having greater stress-bearing capabilities. Grades are determined by the Society of Automotive Engineers and reflected in special markings on the bolt head.

A bolt’s grade can also be indicated by the number of threads per inch it has. More threads mean a finer thread, while fewer threads mean coarse threads. For example, a 1/4-20 bolt has 20 threads per inch, while a 1/4-24 has 24 threads.

SAE bolts can be made of low, medium or high strength carbon or alloy steel. They can also be made of stainless steel, which offers better corrosion resistance than other materials. If a bolt is exposed to heavy-duty conditions, a double nut may be used, with the nuts tightened against each other to lock them in place. This is called double nutting and is particularly useful when a bolt must withstand vibration.