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When choosing spark plugs one has a lot of options. One of the differences is the diameter of the electrode tip - some spark plugs have a large flat tip, and newer models often have a really thin tip. There are many options in between as well, I've seen exact recommendations for certain models.

Fat? Or thin?

As far as I know the spark usually is emitted from the sharp edges of the tip, as the electrical field is the strongest at those points and discharge happens there. Sometimes grooves are added to the tip to increase the length of these edges. So what is the effect of different diameters, is a thinner tip always better?

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Fine wire plugs are less restrictive of the air fuel mixture getting into the gap and the flame getting out. This allows faster flame initiation. Which in turn improves combustion completion percentage. It reduces misfire both complete and partial. From a diver/owner point of view this can improve gas mileage.

The down side of fine wire plugs is that the electrode runs much hotter. Only Iridium has been shown to survive at these increased electrode temperatures.

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