The four things you need to know are your bolt’s:
The thread diameter - is the width across the bolt including the thread.
The overall bolt length - is from the underside of the head to the end of the bolt
The thread pitch size - is the distance between the crests of two adjacent threads.
The thread length - is the length of the thread on the bolt
Most modern bikes (outside of the USA) use metric bolts.
The thread diameter will be given as either M12, M10, M8, M6, M4 or M3. This indicates that they are metric bolts (measured in millimetres) and the number is the width of the thread diameter in millimetres.
So an M12 bolt has a thread diameter of 12mm and so on. The length and pitch are given more simply in millimetres, with the length normally being in whole millimetres e.g. 10mm, 12mm or 20mm and the pitch either in whole or fractions of a millimetre, e.g. 0.8mm, 1mm or 1.25mm.
Imperial bolts are measured in threads per inch, but pitch length is used for standard Metric bolts.
The easiest way to measure a bolt is with digital calipers, which allows you to quickly and really accurately determine the diameter and length of your bolt. To work out the pitch you can use a thread pitch gauge, which is essentially a small metal gauge with a thread of a known pitch profiled into it. These are usually sold in sets covering a wide range of thread pitches. Simply match your bolt to the correct thread gauge to find the pitch.
Don’t worry though. Even though most of you at home probably won’t have these specialist tools, you can still measure your bolts pretty easily and accurately using just a ruler with millimetre markings.
Measuring the length and thread diameter is relatively straightforward, but working out the pitch with a ruler can be a little trickier. To get a really precise measurement, you could measure the space across ten crests, then divide by ten, but as long as your ruler is pretty accurate you can probably safely judge even fractions of a millimetre by eye.
This refers to the size of the pitch and controls how tight a grip the bolt gives you.
Sadly, no, metric and imperial bolts are not equivalent and if you try to force a metric bolt into an imperial hole you will likely end up damaging your bike.
If you are missing a bolt and need to work out what size you need, you can measure the diameter of the hole. This should tell you roughly the width of the bolt you need, then you can carefully check this by trying a bolt of known size into the hole.
The easiest way to check is by trying the bolt in the hole by hand. If it’s loose, or won’t turn easily then the bolt is the wrong size and you need to try a different one. Never force a poorly fitting bolt in and never use power tools if you’re not totally sure you have the right size bolt, as you could ruin the thread and end up with an expensive repair job on your hands.
Racebolt supplies motorcycle bolts both to the racing industry and to individual enthusiasts at manufacturer prices. So if you’re looking for the very best in motorcycle parts, call us on 01179 654 654 or contact us online.