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Rolled vs. Cut Threads

Like fasteners, bolts and screws, a spark plugs shell can be manufactured using various processes. A plug with rolled threads had the threads cast as part of the original steel shell, as opposed to a cut thread, which is machined after the shell is cast. 

For spark plugs, rolled threads are the preferred manufacturing method.  Since the threads are cast and hardened along with the steel shell, the grain structure obtained in the hardening process is not interrupted, creating a higher structural integrity. Rolled threads are often smoother and believed to have a lower likelihood of seizing.