The length of the centre electrode and it's insulator from their 'root' within the plug, does indeed dictate how hot the business end of the plug runs, but the bit Matt is refering to, is the amount that the centre and side electrode protrude beyond the plug outer body, and into the combustion chamber. Plugs of the same heat range, and thus total length of electrode/insulator, are available with, or without the pretruding tip.
clear as mud
PS. yeah, can't remember which way round it is, but NGK and Champion's heat range number work opposite to each other.
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