Spark plug thread size question

GoldenMotor.com

72tx340

New Member
Mar 24, 2010
166
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Caribou Maine
Im going all retro with my cdi. Im going to change to all cloth wire's plus cloth spark plug wire. Im also losing the orginal spark plug cap for a thumb screw type connection.

Now what is the thread size on the top of the spark plug? Im sure its metric but what size?