Among the many different parts which make up the modern machine there are few which are as important or as vital as the nuts and bolts that hold everything together. It is essential that these are the right size so that they will fit the holes into which they are fitted and not get loose under load. This article will explain the different proportions and dimensions associated with metric bolt sizes which will enable you to identify the correct replacement part for any given job.

When you see a bolt marked with the letter M followed by a number, this indicates that it is a metric bolt size. It is important that you know this before buying any metric fasteners as there are a lot of different sizes and it would be very easy to purchase the wrong one.

Metric bolt sizes are designated by their shaft diameter which is measured in millimetres and not in inches like the imperial bolts. The number after the M indicates the thread pitch which is the distance between two adjacent threads and is measured in millimetres as well. The number M8 is indicative of a bolt with an 8mm shaft diameter and an M8 thread pitch.

Bolts are normally made from an alloy of elements such as Iron and Carbon but they can also be made from Alloy Steel which is strengthened by additions of other metals such as Nickel, Chromium etc. This makes the bolt stronger and more resistant to corrosion than plain carbon or iron alone. They can also be galvanized which further protects them from rusting.

There are a lot of different types of bolt available and they are all used in specific applications. Some of the more common ones include hex, cheese, wing and low socket head bolts. The hex bolts are most commonly found on bicycles as they can be tightened and unscrewed with standard allen wrenches.

The head of a hex bolt is usually shaped into a hexagon and the threads run from the face of this to the tip. This makes them easy to grip with wrenches. Hex bolts can be either fine or coarse threaded. Fine threads are more compact and are easier to manufacture because less metal is cut away in making them. However, they tend to be less strong than coarse threads.

If you have a hex bolt with fine threads and it is subjected to a heavy load, it is possible that the threads could start to stretch under the tension. This is known as shear stress and it is important that this is not allowed to happen. The standards generally specify a ‘proof load’ which is 90% of the maximum load that the bolt can be expected to carry so that it will not shear or break. Most manufacturers build machinery with the bolts carrying a far lower proof load than this to provide a safety factor in the design. M8 bolt size