What is the role exactly? Lets say for example i put a studded terminal spark plug on a non loose spark plug in a vehicle, what would happen? Does it effect the way the current and voltage flow?

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The terminal nut it used to change the way the spark plug wire (high tension lead) attaches to the spark plug. The nut affords easy conversion to the appropriate wire type.

According to NGK;

If a spark plug is used for both Terminal nut and threaded stud connection types, it may come with a removable Terminal nut. In this case the plug is manufactured with a threaded Terminal stud, then a screw-on Terminal nut is added to the plug or package. This allows the plug to be used with either connection type.

The wire of one type will not work with the other because it will not make a proper connection. Improper connections cause loss of voltage (weak spark) and/or misfire.

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