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Rolled Thread is the process in which the threads are cast as part of the original steel shell vs. a cut thread which is machined after the shell is cast. Which process is better depends on application. For spark plugs, rolled threads are better as the threads are cast and hardened along with the steel shell, also the grain structure obtained in the hardening process is not interrupted, having a higher structural integrity. Rolled threads are often smoother and believed to have a lower likelihood of seizing.
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