Telling the difference between a metric and imperial bolt is easy. If the bolt has lines on the head its standard or imperial. If the bolt has numbers on the head then its metric. These markings indicate how strong the bolt is they call this tensile strength. With an imperial bolt the more lines you have the higher the tensile the stronger the bolt. With the metric bolts the higher the number, the higher the tensile, higher the strength. Bolts are classified by the diameter of the thread not the diameter of the head of the bolt. Now just by looking at the bolt head you’ll know whether you need a metric spanner or an imperial spanner.
Say you want to know what kind of metric bolt you have or what kind of Imperial bolt. To explain this I’m going to use this snap-on re-threading kit. This table shows the different thread chasers that are in the kit. Highlighting the 3/8 course bolt the thread pitch is 16 threads per inch. This means if you take your bolt and measure
an inch, then count all the peaks of the thread you’ll get 16 in that inch and that’s how you get a 3/8 coarse bolt.
For the metric bolt they simply classify it by the millimeters between the threads. 10 millimeter diameter that’s how you get an m10 by 1.5 . If you wanted to look at it the same way that you look at an imperial bolt you can measure 10 millimeters, count the threads. Then divide 10 millimeters by how many threads you have.
Instead of trying to use a ruler and Counting the threads you can use a thread pitch gauge or I use the thread files in the kit. These have the different thread pitches written on them and it’s just a matter of matching them up. What I find even easier is to grab your bolt look at the head to be able to tell if it’s metric or imperial. Then grab the thread chaser that you think will fit it. Screw it on, then the thread chasers have the classification written on it for you. This is particularly handy for nuts because it’s hard to get a thread pitch gauge or you can’t get a thread file in there and it’s also difficult to measure the diameter.
Apart from being great for working out what type of nuts or bolts you have the rethreading kit is also really good for as it says rethreading. They are designed to push your thread back into shape rather than if you used a tap or a die it cuts away the metal. I often use them if the bolt has been bashed around a bit. Or it has a little bit of rust on it. Flywheel bolt holes are a good one when you’re bolting your clutch up to your flywheel you want to get the correct tension on it. If you run a thread chaser through the bolt holes first you know that the threads are nice and free so you’re going to get the correct torque. If you do get one where the thread is damaged it’s always a good idea to check with your thread file to make sure that you’re using the right thread chaser. Because you will need to use a spanner to turn the thread chaser on to push the threads back in. So making sure that you have the correct thread chaser before you start using a spanner on it is a really good idea. Some metric and Imperial threads are similar, they’re close but not quite. Make sure when you’re using your thread file it lines up exactly.