Nuts and Bolts; Standards, Sizes, Explanations

Nuts, bolts, and screws are very important machine elements in mechanical design. Nearly, all the mechanical stuff has one of these elements. There are different standards, sizes, and procedures to design nuts and bolts systems. Also, screws and studs have a very important place in machinery.

General sizes of nuts and bolts
Image Source: Wikimedia.

Here, you will find detailed information about the nuts and bolts, screws, and studs in different kinds of standards.

Nuts and Bolts

Bolts are the type of machine elements that have a cap and threads at the end of it. We generally put it inside a hole and attach a nut at the end of the threads. So, tight mechanical assembly takes place.

There is another important machine element which we call it cap screw. People generally confuse the cap screws with bolts. The difference between cap screws is, that the threads are longer. And we attach the threads inside the work parts. So, there is an assembly between the work part and the cap screw, unlike the bolts and nuts.

Standard Sizes

Standard sizes of nuts and bolts.

Nearly in all countries, people are using the same mechanical systems. So, parts and components must comply with the standards. People can use the same components in all countries. So, nuts and bolts have standards.

In general, there are different types of bolts. These types are generally regular bolts and heavy-duty bolts. We use heavy-duty bolts for easier wrenching.

The shapes of the heads of the bolts also change. The most common shapes are square and hexagonal heads. These shapes are generally used in both heavy-duty and regular types.

In the metric system, there are hexagonal bolts and nuts in general. Both of them are generally used for heavy-duty and regular jobs.

General Size Proportions

Both in American and Metric systems, bolts and threads have general size proportions. Their proportions are generally according to the diameter of the bolt.

  • Regular Bolts

Width across flats; It is 1.5 times of diameter.

Head height of the bolts: 2/3 times diameter.

Height of the nut: 7/8 times the bolt diameter.

  • Heavy-Duty Bolts

Width across flats: 0.5xd + 1/8 inches (or 3 millimeters)

Head height of the bolts: It is equal to 2/3 times the diameter.

Nut height: It is equal to the diameter.

  • Threads

There are also sizes and proportions of threads.

Thread length for bolts and screws below 6 inches: Two times the diameter plus 0.25 inches or 6 millimeters.

Thread lengths for bolts and screws over 6 inches: Two times the diameter plus 0.5 inches or 12 millimeters.

How to Specify Nuts and Bolts on Technical Drawings?

Technical drawing.

For lots of the machine elements, we have specific specifications for them. For example, we have specifications for screw threads. Also, we have specifications of nuts and bolts according to the standards. 

In a general specification; we are adding the nominal size of the bolt body, thread specification, length of the bolt, surface finish of the bolt, head style of the bolt, and name of the bolt. If you give this information respectively, you will obtain a neat nut and bolt specification. Check the examples below.


This is a general example of a bolt. Here, .5 defines the nominal diameter of the bolt. 5UNC-2A is the specification of the thread group. You can learn how to specify threads at the given links above. X 2.5 is the length of the bolt. As ‘STAINLESS STEEL HEX. CAP SCREW’, you can see the name of the bolt screw.

  • M12 – 2 X 20 SS HEXCAP SCR

Above, you can see an example of a metric bolt specification. Also, the name is abbreviated. So, you can fit this name on engineering drawings.

‘M12’ metric nominal diameter of the bolt. -2 means the thread size. X20 is the length of the bolt in millimeters. SS HEXCAP SCR is the abbreviation of the ‘STAINLESS STEEL HEX. CAP SCREW’.

If there is an extra not on the abbreviation such as ‘HEAVY’ or ‘HIGH STRENGTH STRUCTURAL’, the type of the nut or bolt is regular. Otherwise, it is a heavy-duty nut or bolt.

Specifications of Nuts

There are also specifications of nuts. Check the examples below to understand it.

  • 5/7 – 10 UNF -2A SQUARE NUT

As you see above, the specification is nearly the same with bolts. 5/7 is the nominal diameter of the nut in inches. 10 UNF -2A is the thread specification. SQUARE NUT defines the type of the nut. You can also use the abbreviation of it such as SQ NUT.

  • M18 – 2.5 HEX NUT

This is a metric example of the specifications of nuts. M18 gives the information about the nominal diameter of the nut in millimeters. 2.5 is the thread size. And HEX NUT is the general classification and type of the nut.

Grades of Hexagonal Bolts

Grades on the bolt caps.

Both metric and SAE hex bolts have grades on the cap. For metric bolts, there are numerical on the cap such as X.Y. This is giving information about the strength of the hexagonal bolts.

For example, class 8.8 grades of metric bolts have a maximum tensile strength of 800MPa. And also they have maximum yield strength of 640 MPa. They are produced from medium carbon steel and they are quenched and tempered.

For US bolts, there are radial lines that give important information about the strength of the bolts. For example, if there are 6 lines on the cap, the material is medium carbon alloy steel. This bolt is quenched and tempered. The maximum tensile strength is 150.000 psi and the maximum yield strength is 130.000 psi.

Metric Bolts

A2 and A4 Stainless: This is the class of metric bolts with the A2 and A4 stainless quenched and tempered steel material. The maximum tensile strength is 500MPa and the maximum yield strength is 210 MPa.

8.8 Class: The material of the 8.8 class metric bolt is medium carbon steel which is also quenched and tempered. The maximum and tensile strengths are 640 and 830 MPa respectively.

10.9 Class: You can see the 10.9 phrases on the cap of the bolt. This class is the alloy steel which is quenched and tempered. The maximum yield strength of this class is 940 MPa and the maximum tensile strength is 1040 MPa.

12.9 Class: Also there is a 12.9 phrase on the bolt cap. The material is the same as the 10.9 class. And the maximum tensile strength of this class is 1220 MPa and the maximum yield strength is 1100 MPa.

US Bolts

Grade 2: At the top of the hexagonal cap, there are no markings for this grade. The material of the grade 2 bolts is low or medium carbon steel. The maximum yield strength of 36.000 psi and the maximum tensile strength is 74.000.

307A: This is a special grade of US Bolts. The material is low-carbon steel. Also, the maximum tensile strength of this grade is 60.000 psi.

A325: This grade is carbon alloy steel with boron. The maximum tensile strength is 120.000 psi and the minimum yield strength is 92.000 psi.

Grade 5: At the top of the hexagonal cap, there are 3 line symbols. Grade 5 bolts are medium carbon steel that is quenched and tempered. The maximum yield strength is 81.000 psi and the maximum tensile strength is 105.000 psi.

Grade 8: It has six lines on the cap. Grade 8 is medium carbon alloyed steel which is quenched and tempered. The maximum yield and tensile strengths are 130.000 and 150.000 psi.

Aluminum 2024: This is a special grade of US bolts. The material is aluminum alloy with copper. Also, it is alloyed with magnesium and manganese. The general processes are solution heat treatment and age hardening. The maximum tensile strength is 55.000 psi and the maximum yield strength is 36.000 psi.

651 Silicone Bronze: Bronze includes a small amount of silicon. The maximum tensile strength is 70.000 psi and the maximum yield strength is 40.000 psi.

Locknuts and Other Locking Parts in Nut and Bolt Assemblies

Unscrewing is one of the most important problems in the nut and bolt assemblies. Because of the external effects such as vibrations, nuts can get loosen. To prevent this situation, there are different locking parts and locknuts. We are using such assemblies to prevent the unscrewing phenomenon.

Locknut application.

Locknuts are the additional second nuts that we attach after the nut. It prevents the unscrewing of the nut. In general, the thickness of the locknuts is lower than the nuts. They are generally half of the diameter of the nut.

In the heavy types of locknuts, there are washer faces. We use washers to spread the total load over the surface.

Set screw and cooter pins.

Cotter pins are other kinds of locking parts. They are generally made from more ductile materials and attached to nuts to prevent unscrewing.

Also, set screws are other important parts to prevent unscrewing. So, we need to unscrew the set screws and cotter pins to disassemble the nuts and bolts.

Advantages and Disadvantages of Nuts and Bolts

We are using nuts and bolts for assembly operations of different parts. There are also other types of assembly such as welding, riveting, etc.

But the most important advantage of the nuts and bolts is that they can be attached and detached. But in welding and riveting, if you try to detach, the main work parts can be damaged.

For example, you need to detach the surface sheet metal of machinery to mend it. The surface is assembled with bolts and nuts to the machinery. So, you can disassemble the sheet metal to mend the machinery. But if it was a rivet, you may harm the sheet metal parts. Also, reassembling is very hard for it.

Mechanical strength can be a problematic issue for nut and bolt assemblies. Because the shear forces can be very high for bolt assembly. But if we think about welding, the force will spread on the weldment area, and stresses will be lower.

These are the general advantages and disadvantages of the nuts and bolts mechanisms.


Above all, this is the general information about the nuts and bolts. Nuts and bolts are produced in standard sizes and dimensions. These standards are very strict. Also, you need to show them which standard your bolt and nut fall to. Also, there are specific types of bolts and nuts according to the standards.

Grades are also very important for bolts and nuts. You need to know which type of grade you are using. Because in the strength calculations, you need to use the proper one.

In general, designers are using locking elements and locking parts for the nuts and bolts assemblies.

So, do not forget to leave your comments and questions below about nuts and bolts.

Your precious feedbacks are very important to us.

FAQs About Nuts and Bolts

How are bolts and nuts made?

There are different methods to produce bolts and nuts. The main cylindrical geometry is obtained with forging and shaping operations. And then, they apply basic turning and machining operations to obtain threads.

What are the bolts and nuts made of?

In general, steel is the most common material. The alloying elements change with the changing classes. Also, aluminum alloys and bronze bolts, and nuts are among the other applications.

How to tighten nuts and bolts?

It is very easy to do. You need to hold the bolt from the head with the help of a wrench. And you need to attach the nut with another wrench.

Can the bolts and nuts recycle?

Yes, they can. We can produce steel from scratch bolts and nuts from scratch machinery.

How to order and buy nuts and bolts?

It is very simple to do. Because nuts and bolts are standard shapes and it does not matter which seller you will buy. You can search for the bolt and nut sellers on Google to find the nearest sellers. But in general, you can find the most common ones from hardware stores.


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