Humble bolts seems like a mundane and boring machine component. However, we engineers always get a little excited at threaded fasteners, simply for the incredible variety available and their significant capability in any engineering application. You see them in bridges and buildings, aircraft, cars and of course, our motorcycles.
Why would you choose to use threaded fastener? Usually, for flexibility in assembling or disassembling components. Sometimes the cost of using a threaded fastener to secure a joint might be cheaper than other options (such as welding a joint together). In our motorcycles, there will be a combination of reasons.
A threaded fastener (lets just simplify and say “bolts” from now on, it will save me typing “threaded fastener” another 100 times) works like a spring. Remember our discussion in the Basic properties of metals article about the tensile test. A bolt works in tension (meaning the load is parallel to its long centreline axis). When you put a bolt through, say, two pieces of steel, and tighten the nut, the steel plates do not compress appreciably, so the distance between the nut and the bolt head is essentially fixed. However if the bolt is being tensioned, it is stretching between the nut and the bolt head, which are up tight against our theoretical steel plates. If the bolt is stretching, then it is exerting an equal but opposite clamping force on the two steel plates. This diagram might help illustrate the point.
It is this clamping force which is important. The clamping force is intended to prevent any slip between the parts. The amount of operational forces the engineer expects the assembly to withstand in service, will, in turn, dictate how much clamping force is required between the parts. This clamping force will then inform the bolted joint design. How many bolts can I fit on this assembly? What size bolts can I fit here? Are there particular geometry features that I have to consider which limits my bolt choice? Do I have to change the shape of the part to fit enough bolts into the assembly?
Over the industrial age through to modern times, various thread forms have been developed by different industries in different countries. This is what they look like in cross section: