M8 Bolt Size Dimensions [What Does M8 Mean]

Bolts are one kind of threaded fastener that has a male thread on the outside of the buckle. This thread matches a pre-formed female line, such as a nut.

In addition, the bolt is a generic name for a fastener in which just a portion of the shaft is threaded. A screw is a fastener with its entire post threaded. Take note; the bolt’s shaft’s unthreaded section is known as the shank.

Nonetheless, we have different bolt sizes because they vary in specification and uses. Sizes of bolts can be expressed in millimeters, but its measurements are most commonly used in inches.

According to Science Direct, “Threaded fasteners are used for many applications, so although their function is simple, there is a wide variety of designs and sizes.”

So, in this article, we will help you to know what is an M8 bolt. The size and specification of this bolt are important factors. Keep on reading to learn more!

Figuring Out Metric Bolts

Metric bolts are sometimes referred to by M-size numbers, such as M3, M8, and M12. For more exact metric fastener measurements, use the metric fastener’s dimensions of diameter, pitch, and length in millimeters.

For the size of nuts, the circumference and pitch are the only measurements needed. Meanwhile, this size measurement is different from the American measurements of bolts.

American bolts are created in units of measurement known as inches in the United States customary system. Even if the global market uses metric sizes, the US still uses different ways to measure the bolts.

The American and metric unit standards have been adopted because of the issuance of standards from American standards groups. Also, several bolt naming schemes and various hardware dimensions have led to variations in unit usage.

How To Read Metric Bolt Size and Specifications

M8 Bolt Size Dimensions

The size of a metric bolt is given in millimeters in terms of diameter, pitch, and length. Nuts have a diameter and pitch associated with their size. If the angle is not provided, coarse-threaded fasteners are the exception.

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Here is an example of metric bolt size:

M8-1.25 x 20

The description of this bolt says:

  • This bolt utilizes a metric thread, and the letter ‘M’ denotes this factor.
  • The number indicates the bolt shaft’s nominal diameter of ‘8’ millimeters.  
  • The bolt’s thread pitch is 1.25 mm, or the distance between threads.
  • The last measurement is in millimeters, and it is 20 millimeters in length.

Everything About M8 Bolts

For you to understand well what an M8 bolt is, size and specification must go first. Here are some of the specifications that you should take note of before purchasing one.

1. Metric Thread Pitch

If you operate with standard inch fasteners, you may find that pitch is difficult to understand. Take note; the gap between two consecutive threads is called pitch.

In the example shown above, 1.25 mm is the pitch between the thread of one needle and the line of the next needle. As opposed to a thread per inch, also known as the pitch, inch fasteners are defined as threads per inch.

It is easy to know the threads per inch of your metric bolt. First, translate the metric thread pitch in millimeters to inches (multiply by 0.03937). Now divide one by that figure.

For a metric bolt with a size of M8-1.25 x 20, the pitch in inches of it will be 0.0492. Therefore, the threads per inch of this metric bolt are 20.32.

2. Metric Bolt Lengths

Measuring the length of bolts is the same, except in millimeters. You should measure flat-head screws, socket heads, cheese, hex, pan, and low socket heads from behind the head.

Moreover, the height of flat-headed animals also contributes to their overall length. Meanwhile, to measure the ovals, start by finding the biggest diameter point and finishing at the tip. Remember, the diameter point is the dome top connecting to the conical bearing surface.

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Furthermore, standardized metric fasteners are widely produced to fulfill DIN and ISO specifications.

3. M8 Standard Hole Sizes

The metric bolt’s diameter is slightly greater than the shaft diameter. The shaft diameter of an M8 bolt is a little under 8mm. Hence, the bolt will easily fit through an 8mm hole.

But clearance holes are often drilled a bit larger to allow for misalignment. When it’s time to clip a thread, you should reduce the hole diameter to avoid poking the line.

Determining the requisite tapping hole size is simple when the metric bolt diameter is subtracted from the thread pitch. A usual coarse thread pitch of 1.25mm would mean a 6.75mm diameter hole would need to be tapped for an M8 bolt.

However, if the M8 bolt had a thread pitch of 1 mm, the resulting hole diameter would be 7mm.

1. Clearance Hole

A clearance hole is a space in a component that allows a threaded bolt to pass through. So, here are calculations that present the suggested drill bit size for each class of clearance hole. Also, there are numbers as the nearest alternative in inches.

  • Nominal Metric Drill Size: 9 mm
  • Nearest Inch Drill Size: T in
  • Minimum Hole Diameter: 9 mm
  • Maximum Hole Diameter: 9.22 mm

2. Tapping Drill for Coarse Pitch

For an M8 bolt, the standard coarse thread pitch is 1.25. In line with this, the corresponding rough pitch tapping drill size for it is 6.75 mm.

3. Tapping Drill for Fine Pitch

An M8 bolt has a fine thread pitch of 1.0. For this reason, the tapping drill for a fine pitch of this bolt is 7 mm.

4. Standard Spanner and Allen Key Sizes

It is always important to use the right tool for the job. To prevent slipping, you must use the proper size spanner or Allen key. It also avoids dulling the bolt by rounding off the edges.

It will help if you realize that every bolt has several variants, including Allen bolts, hex bolts, and more. For proper removal of fasteners, the tools used should be the right size.

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Take note; we use Spanners or Wrenches for hexagonal bolts. For the Allen bolts, Button Head bolts, CSK bolts, and Grub screws, we have an Allen key or Hex key.

So, here are the Spanner and Allen Key sizes for M8 bolts:

  • Spanner/Wrench for Hex bolts: 12 – 13 mm
  • Allen/Hex Key for Allen bolts: 6 mm
  • Allen/Hex key for CSK bolts: 5 mm
  • Allen/Hex key for Button head bolts: 5 mm
  • Allen /Hex key for set/ grub screw: 4 mm

5. M8 Bolt Maximum Torque Thresholds

As a rule, bolts are only utilized in rare instances. Furthermore, the manufacturer’s maintenance manual should have the torque requirements listed.

Fine threads are commonly utilized in aluminum castings when bolts are used to screw into soft metal blocks. While the bolt may provide a firm connection, the line in the block may be quite weak.

 As a result, a lower torque is needed to avoid thread stripping.

Thread strength is a function of a variety of things. It all starts with the thread’s raw materials, its surface quality, and the thread’s accuracy when it comes to shaping.

As with other types of connection, the strength of the threaded connection depends on the power of the weakest component.

So, if a high-tensile bolt is screwed into a more pliable material, the more malleable material will be the section that gets penetrated. The maximum torque that you can safely apply to the bolt is decided by it.

A threaded hole created by drilling and tapping may not be the same size as the hole you drilled. If the drill is well-sharpened and aged, it will rely on how successful it is. There’s a good chance the gap isn’t straight, and the tap is rough or dull.

This combined effect will yield a thread that doesn’t quite fit the bolt the way it should. As a result, the joint could be impaired.

For design engineers, all these considerations are relevant, and they may select a lower torque. In lifting devices, it is often the case that strong safety factors are needed.

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Remember, always go to the maintenance manuals provided by the machine maker for bolts. Plus, use the prescribed torque.

5. M8 Standard Proof Loads

If a bolt is loaded gradually, it will extend and then break at some time. The proof load is established as the basis for how much gear a bolt should be authorized to bear.

Take note; around 90% of the force that causes the bolt to expand occurs with 90% of the weight on it. Proof load is generally thought of as the maximum load that a bolt should bear.

Most manufacturers build their machinery such that the nuts rather than the bolts carry a proof load. It will provide a layer of security to the design.

In addition, 10% more robust fine-pitch bolts are usually compared to coarse-pitch bolts. The thread is easier to manufacture since less metal is removed.

Frequently Asked Questions (FAQs):

What Does A Metric Bolt Size Of M8 Mean?

The “M” denotes metric. Then, the number indicates the nominal diameter of the fastener, which may be 6mm, 8mm, or 12mm, for example.

Typically, screw size designations such as M8x1.0 are used. The value “1.0” denotes the thread pitch or the distance between two threads’ lands or grooves.

Is An M8 Bolt 8mm Wide?

An M8 bolt has a width of 8 mm. However, holes and drilling do not follow this 8 mm bolt.

Larger clearance holes frequently allow misalignment. Reduce the hole diameter to avoid pinching threads.

From the metric bolt diameter, subtract the thread pitch to get the tapping hole size. For a 1.25mm coarse thread pitch, a 6.75mm diameter hole is needed.

What Size Bolt Does A M8 Nut Fit?

An M8 nut needs an M8 bolt. The difference in bolt and nut type (M8 versus M8) depends on threading.

To check if your fastener is a fine or coarse thread, you’ll need to do that. Then use the proper M8 nut for it.

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The 13-millimeter wide hex nut is the standard M8 size. Furthermore, nearly all nuts will be this size. But, you will sometimes encounter a variety of other measures, particularly with mass-produced items.

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It can be somewhat more or less tall, depending on the nut type. Thread depth will be around 80% of the nominal diameter of the bolt.

Nyloc and dome nuts are slightly taller than standard nuts. You can also acquire half-height nuts, which are utilized with conventional nuts for locking or are used on their own.

What Is M8 In Standard?

You’ll find a metric M bolt measuring 8mm across. It is about the same size as a 5/16 screw or bolt.

It is common practice to relate the specification to the coarse pitch without a metric pitch.


Metric fasteners can help you to have a more concrete project. Do you want to answer if what is an M8 bolt? The size and specification of this bolt can guide you to understanding it.

For your next task that involves an M8 bolt, use this article to assist you. Remember, you need the right size to avoid any problems. Do not get overwhelmed with the various bolt and nut sizes out there.

After all, your high-end tool deserves the right and proper fit. So, do not settle for less!

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