Few parts are as critical to the operation of modern machinery as the nuts and bolts that hold it together. For those who work in plant and machine maintenance, it’s important to have a basic understanding of the ISO metric system of bolt sizes so that when replacement bolts are needed they can be identified correctly.
M8 bolt size is a common metric bolt diameter and thread pitch used in various applications. This bolt size is not as commonly used on bikes as M6 and M5 bolts, but they are found in some applications such as quill stem bolts and seat post bolts. It has a thread length of about 125mm and can be screwed and unscrewed with 6mm allen wrenches or 13mm hexagonal wrenches.
Metric bolts are sized according to an ISO standard that is based on diameter and pitch (the distance between threads). The first number indicates the outer diameter of the bolt, while the second number indicates the number of threads per millimeter that are in the bolt. Bolts are also rated according to their tensile strength. This is indicated by the ‘grade’ marks that appear on the head of the bolt and is a measure of the amount of load that a bolt can carry before it fails.
The higher the grade, the stronger the bolt. When a bolt is loaded, it will extend slightly and then return to its original length. However, there comes a point at which the bolt will start to yield or permanently stretch and if the load continues to increase there will come a point at which the bolt will fail – this is known as the breaking load of the bolt. The standard grades of hex bolts are 8, 10, and 12.
There are a few ways to determine the bolt size, including using a tool called a thread gauge or thread checker. This can be purchased online or at most hardware stores. A caliper can also be used to get an accurate measurement of the bolt. In general, a bolt with a coarse thread will require a larger hole than a fine threaded bolt.
When working with metric bolts it is important to know what the thread size and pitch are so that the correct nut can be used. The easiest way to do this is by dividing the smallest lines on a tape measure into sixteenths and counting the number of lines that are divided into these sixteenths. A chart can also be downloaded from the ISO website that shows a table of clearance holes, standard and fine thread pitches and their corresponding tapping hole sizes. This chart is useful for determining the bolt size and specification for any application. Generally, it is recommended to use a nut of the same grade or one grade higher than the bolt for best results. This is because a higher grade nut will have better grip and prevent the bolt from loosening over time. M8 bolt size