On the white part of spark plugs there are those colored rings. In amount ranging from 0 to 4.

What does the amount of rings refer to? And what does it's color refer to? Is it just aesthetics?

Different spark plugs

  • I wonder if it like resistors except thermal resistance by length of elect. insulator which is a heat radiator May 7, 2019 at 14:39

2 Answers 2


I believe it is the "heat range" of the plug. That determines the temperature of the electrode in service. Changing the length of the center electrode affects how fast heat is conducted away from it. Long ago when many cars burned oil , a "hot" plug was recommended for a oil burner to burn carbon off the electrode.


For some brands its only aesthetics for other it marks what type of electrode spark plugs have. Also as far as I know Bosch green ones are replaced by blue ones.

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