What Is an SAE Bolt Thread Size?

In the engineering world, having standardized bolts and fasteners are extremely important. They allow workers all around the world to know exactly which bolt works where and if needed, they can replace them with ease. This standard sizing is done by SAE, who have set up one of many standards. This sizing is also maintained. There are other kinds of standards as well, namely the UNF, the USS, and more. 

SAE thread sizes come in a variety of sizes, and the number associated with each measures the number of threads that are present on the fastener. The more threads the fasteners have per inch, the finer the threads are. These threads also have an impact on how the bolt or fasteners fit. However, it also depends on the use which determines the kind of bolt you will need with what number of threads. 

Let us take a deeper look into bolt threads and specifically SAE bolt threads.

What are bolts and bolt threads?

What are bolts and bolt threads

Bolts, nuts, and fasteners are extremely important in the engineering world. This is because they are used in most compound machines these days. Without these machines, modern society would be in shambles. Hence, these small things that seem insignificant at first are crucial. In modern times, thread fasteners are manufactured to help meet the demands of robust yet highly efficient machines. 

A bolt is a kind of a threaded fastener that includes an external thread. This thread fits into a matching pre-formed threat which can be a net. Bolts are very similar to screws. However, the difference is minute. According to Machinery Handbooks, the main purpose of bolts is to pass through an unthreaded hole in the component that it is meant for. Furthermore, they can be fastened using a nut. Fastening can be done without a nut as well. Screws, on the other hand, are used in places that have their own thread. Bolts also have a variety of different head types. These include hex, socket cap, and slotted hex washer. 

A screw or bolt thread is a helical structure that allows you to rotate and push in the screw or bolt into its intended compartment. This thread usually appears as a ridge wrapped around a cylinder or a cone. The configuration is in the form of a helix. When on a cylinder, it is called a straight thread. When on a cone, it is called a tapered thread.

These threads are incredibly important, and the importance depends on the pitch of the thread. A pitch is essentially a linear path a screw covers when it twists one complete revolution. There are four standard kinds of threads. These include UNC threads, UNF threads, and 8-UN threads. The UNC is considered a coarse thread, the UNF is a fine thread, and the 8-UN has a total of 8 threads. There are different grades of bolt threads available as well. 

What does the SAE stand for?


SAE stands for Society of Automotive Engineering. It is one of many standards that the engineering world has established. There are different kinds of threads available on the market that have different kinds of threads. Such variety allows there to be bolts and screws available for a variety of different purposes. There are a total of six different kinds of threads, and some are used more than others. These six types include the UN or UNF, BSPP (BSP, parallel), NPT or NPTF, metric parallel, BSPT (BSP, tapered), and the metric tapered. Out of these, there are three parallel and three tapered. 

The UN/UNF, BSPP (BSP parallel), and metric parallel are the parallel kinds. 

The BSPT, NPT/NPFT, and metric tapered are the tapered varieties. 

Among the SAE bolts, there are two general kinds. They are the UNF and UNC. The UNF has a fine thread pitch and has more threads per inch as compared to the UNC. The UNC is a coarse thread pitch. The UNF is mostly used in precision applications because they have larger tensile strength areas. They also have a high tensile strength which is ideal for the products they are used in. The UNC is mostly used as general nuts and bolts. The reasoning behind their usage is that they are less likely, as compared to other threads, to cross thread. They also have more strength and are overall more tolerant of adverse conditions. Furthermore, they are easy to use. 

SAE International is a professional association and standard development organization. Their specialty lies in the engineering industry, and they focus mainly on the transport sector. Therefore, automotive, aerospace, and other commercial vehicles come under their sphere. SAE is well known in the automobile horsepower and aerospace industry. Therefore, their standard sizes adhere to those sectors. 

Common SAE bolt thread sizes

Common SAE bolt thread sizes

The table below shows all the commonly used varieties of SAE bolts.

0-80 UNF

1-64 UNC

1-72 UNF

2-56 UNC

2-64 UNF

3-48 UNC

3-56 UNF

4-40 UNC

4-48 UNF

5-40 UNC

4-44 UNF

6-32 UNC

6-40 UNF

8-32 UNC

8-46 UNF

10-24 UNC

10-32 UNF

12-24 UNC

12-28 UNF

1/4-20 UNC

1/4 -28 UNC



3/8-16 UNF

3/8-24 UNC



1/2-13 UNF

1/2-20 UNC

Here is another table that provides more information on bolt threads

ThreadAluminum, Brass & Steel Nylon (‘L’)Stainless Steel (‘L’)
1/4-201 1/41 1/4
1/4-281 1/41 1/4
5/16-181 1/41 1/4
5/16-241 1/41 1/4
3/8-161 1/41 1/4
3/8-241 1/41 1/4
7/16-141 1/41 1/4
7/16-201 1/41 1/4
1/2-131 1/41 1/4
1/2-201 1/41 1/4

Different grades of bolts and their uses

Different grades of bolts and their uses

Bolts available on the market are used in a variety of different places. Hence, they come in a large variety of grades as well. These grades indicate the tensile strength of a bolt and generally whether or not it can be used in a specific field. Here is a breakdown of all the grades available on the market. These grades range from grade 2 and go all the way up to grade 8, which is considered the strongest bolt available under the SAE American Grades

SAE American Grade 2

The SAE American grade 2 is recognized through the lack of markings. It has a tensile strength that is estimated to be around 60,000 and 74,000 psi. This tensile strength makes it quite strong and is thus generally used for joints and applications that are non-critical.

SAE American Grade 5

SAE American grade 5 bolts can be recognized among other bolts because it has three radial lines. This kind of bolt has a tensile strength between 105,000 and 120,000 psi. Because of their high tensile strength, these bolts are used in automotive appliances as they hold up pretty well when compared to other bolts.

SAE American Grade 8

These grade 8 bolts are the most sought-after bolts. They are recognized by six raised dashes that make them stand out among all other bolts. These bolts are made of medium-carbon alloy steel. This material has been both quenched and tempered. Therefore, it has an impressive tensile strength that is measured at 150,000 psi. This when compared to grade 2’s 60,000 to 70,000 psi, there is a massive difference. You can say that grade 8 is almost twice as strong as grade 2 bolts. 

Due to its strength, grade 8 bolts are high in demand in automotive suspensions, which require bolts that can handle great loads. Thus, in such places, these SAE American Grade 8 bolts are perfect.


Bolts, nuts, and fasteners are extremely important in the engineering world. They are literally what is used to hold various different machines together, and are thus crucial for various different industries. SAE or Society of Automotive Engineering is one of many standards that the engineering world has established. SAE bolts come in many different thread sizes, as detailed above. However, the size that you need depends solely on your use. SAE is an effort to standardize the bolts and screws have a chart that lists all their different bolt thread sizes. Other than size, SAE also uses two different kinds of threads, namely the UNF and UNC. Both of these have different uses, and care must be taken when choosing which one you need. Mostly, SAE is used in the automobile horsepower and aerospace industry. Therefore, their standard sizes adhere to those sectors.