You are probably most familiar with cut threads. You take a rod or bolt and cut the thread onto the shank with a die, or a die nut. Naturally this leaves you a thread that is cut from the original material, which means you have some stress raiser points in the teeth of the thread. The form of the thread will dictate how high or low they are.
With a rolled thread you take your shank and force a wheel into the material, squeezing the material up from the surface to form a thread. It often takes several goes at this before you get the finished thread, at the required size. A rolled thread can be likened to a forging because the original grain of the material flows into and along the threaded section, rather than being cut at 90 degrees to the flow of the grain.
Rolled threads are therefore felt to be superior in terms of mechanical strength (size for size) to a cut thread. The best bolts, or at least the more expensive ones, tend to have a waisted section, perhaps with a shoulder at one end and a thicker section half way down if it is a long bolt. The theory is that the bolt will only be as strong as the thinnest cross section at the base of the thread, so the shank need not be any thicker than this. The raised sections are simply there to provide location within the hole, since the hole must be size of the outside of the thread, not the base of it.
On most bolts you will find a number on the top of the bolt. This represents the tensile strength of the bolt. For example, an 8.8 would be a general purpose steel bolt, while a 10.9 is higher tensile. If you are replacing bolts around the car, always check the rating and replace like with like.
(From Technical Tips in the MSA Wheels! magazine supplement, March 2001)
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