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A plugs hex size determines the socket required for installation.
While 5/8" (16mm) and 13/16" are the two most common plug hex sizes, plugs can have a hex as small as 8mm or larger than 1".
If you're a tuner or engine builder in search of the perfect spark plug for your current project, pay special attention to the hex size required. Remember to take into consideration not only the diameter of the cylinder head, but also the added thickness of the plug socket needed for installation.