Bolt Head Size Chart (Metric and Standard)

If you work with small engines long enough, you will eventually encounter a broken or missing bolt. It happens, it’s frustrating, it’s time-consuming, I get it.

Which size bolt will you need to replace it? How do you convert Metric and Standard? What’s the thread pitch? Properly sizing a replacement bolt can be challenging. The bolt head size chart below can help.

What is a bolt head size chart?

A bolt head size chart is a tool that provides a visual representation of the different sizes of bolt heads, typically showing the size of the bolt head in both imperial and metric units.

Bolt head size charts are used as a reference guide to help users select the appropriate size of bolt head for a given application. They can be helpful in determining the size of a bolt head when only the bolt is available.

Bolt head size chart: A quick reference guide

Bolt Size Standard Diameter Metric Diameter
1/4 0.25 in. 6.35 mm
5/16 0.3125 in. 7.9375 mm
3/8 0.375 in. 9.525 mm
7/16 0.4375 in. 11.1125 mm
1/2 0.5 in. 12.7 mm
9/16 0.5625 in. 14.2875 mm
5/8 0.625 in. 15.875 mm
3/4 0.75 in. 19.05
7/8 0.875 in. 22.225 mm
1 1 in. 25.4 mm
1 1/8 1.125 in. 28.575 mm
1 1/4 1.25 in. 31.75 mm
1 3/8 1.375 in. 34.925 mm
1 1/2 1.5 in. 38.1 mm
1 3/4 1.75 in. 44.45 mm
2 2 in. 50.8 mm
2 1/4 2.25 in. 57.15 mm
2 1/2 2.5 in. 63.5 mm
2 3/4 2.75 in. 69.85 mm
3 3 in. 76.2 mm

How to measure bolt head size accurately

Use a bolt gauge: A bolt gauge is a tool specifically designed for measuring bolt head size. It consists of a series of holes of different sizes, and you simply place the bolt head in the hole that fits it best.

Use a caliper: A caliper is a precision measuring instrument that can be used to measure the diameter of the bolt head. To use a caliper, place the bolt head between the jaws of the caliper and carefully adjust the caliper until it fits snugly around the bolt head. Then, read the measurement on the caliper’s scale.

Use a ruler: If you don’t have a bolt gauge or caliper, you can use a ruler to measure the bolt head size. To do this, place the bolt head on a flat surface and place the ruler alongside it, aligning one end of the ruler with the center of the bolt head.

Then, measure the distance from the center of the bolt head to the farthest point on the bolt head.

Use a digital caliper or micrometer: Digital calipers and micrometers are more precise measuring instruments that can give you an even more accurate measurement of bolt head size.

To use these tools, follow the same basic steps as with a regular caliper or micrometer, but be sure to read the digital display carefully to get an accurate measurement.

Nuts and Bolts

Regardless of which method you use, it’s important to be precise and take multiple measurements to ensure that you get an accurate reading of the bolt head size.

Metric bolt bizes, what does the M mean?

Metric bolts have an M stamped on the bolt head followed by a value. This tells you it’s a metric bolt.
Although Metric bolts are measured in millimeters (mm) the M value is a measurement size, not the actual size of the bolt in millimeters.

Use this chart to determine the mm size of the M value stamped on a bolt. Bolt size is measured across the head of hex and square bolts, from flat side to flat side, just like Standard bolts.

  • M3 bolt = 5.5 mm
  • M4 bolt = 7 mm
  • M5 bolt = 8 mm
  • M6 bolt = 10 mm (most common small engine bolt size)
  • M7 bolt = 11 mm
  • M8 bolt = 13 mm (very common bolt size)
  • M9 bolt = 17 mm (very common bolt size)
  • M10 bolt = 19 mm
  • M12 bolt = 22 mm
  • M14 bolt = 24 mm
  • M16 bolt= 27 mm
  • M20 bolt = 30 mm

Bolt size + thread pitch

Thread pitch refers to how tightly the thread of a bolt goes around the bolt shaft. It is absolutely necessary to make sure you use the SAME thread pitch when replacing a bolt.

Using the wrong thread pitch may result in a damaged bolt or, worse, a damaged small engine block. This is because the threads are not the same size and will strip or “cross thread”.

My best advice to avoid buying a bolt with the wrong thread pitch is to bring a good bolt from the engine that fits in the bolt hole to your local hardware store for direct comparison.

Bolt head size standards and specifications

ISO metric screw thread: The ISO metric screw thread is a standardized screw thread system that is widely used around the world.

It specifies the dimensions and tolerances for metric bolt head sizes, as well as the corresponding wrench sizes and other details.

Unified Thread Standard (UTS): The Unified Thread Standard is a standard for screw threads that is used in the United States and Canada.

It specifies the dimensions and tolerances for imperial bolt head sizes, as well as the corresponding wrench sizes and other details.

ASME B18.2.1: The ASME B18.2.1 standard is a specification for hex bolts, hex cap screws, and other types of bolts. It specifies the dimensions and tolerances for both metric and imperial bolt head sizes, as well as the corresponding wrench sizes and other details.

SAE J429: The SAE J429 standard is a specification for bolts, screws, and studs that are used in automotive and related industries.

It specifies the dimensions and tolerances for both metric and imperial bolt head sizes, as well as the corresponding wrench sizes and other details.

In conclusion…

Take your time, don’t force anything and start threading your bolts with your fingers, they shouldn’t be hard to turn at first. If they are, you may have the wrong size of bolt.