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Measuring a Bolt / Screw

What is the Diameter of the Bolt / Screw?

Measure this using a vernier if possible - if you are using a ruler do your best to measure outside of the threads. Do not measure the head of the bolt, measure the threaded part, outside of the threads. If your bolt is metric, then the diameter is stated using the letter "M". For example, if your bolt has a diameter of 4mm then it is called an 'M4 bolt' if your bolt is imperial then it will be measured in inches. for example if it measures 1/4", then the bolt diameter is simply 1/4".


Which Thread do you Need?

The 'thread' of a bolt is calculated by the distance and angle between the ridges, or on most imperial bolts you will need to count the number of threads per inch. Ideally you will use a thread guage to do this.

However most of the general public do not own these valuable tools. Measuring with a screw pitch guage if you do not have a thread guage, try to follow these steps:- What is the bolt on? Is it a European car? An American car? A general rule of thumb is that European cars use Metric threads and American cars use Unified threads. If your bolt is not used on a car think about where and when it was manufactured. Many products are now manufactured in places such as China and Taiwan and therefore are most likely to use Metric threads. Please note that this is just a rule of thumb. In some cases a company that is manufacturing a product or a vehicle will simply use whatever they can find that is cheap. Older vehicles may also use different threads In the past threads such as BA and BSF (both British threads) were more popular. if you are unsure it is well worth investing in a thread guage or you can post a sample to us and we will measure it for you.

What is the Length of the Bolt / Screw?

Measure the length from underneath the head to the end of the bolt. This includes the shank, if your bolt has one. A shank is an unthreaded part underneath the head. If you are measuring a countersunk screw make sure to measure the entire length.


Which Material?

In most cases it is best to replace like with like. If your bolt is high tensile grade 8.8 steel then usually you will need to replace with the same or stronger. Think about where you are putting the bolt. Will it be under strain? Is it just there to look nice? On more than one occasion we have had customers wanting to buy stainless, which is weak, to replace high tensile bolts used in suspension. This is dangerous. If you are unsure then speak to a specialist. We are happy to provide advice but it is your responsibility to purchase the correct item.