After much deliberation and investigation, the Association of Licensed Automobile Manufacturers has decided upon a standard spark plug.
The spark plug adopted has a seven-eighths inch diameter, straight thread, and is 18 inch pitch. The stock of the plug is one and one-eighth inch around. The head form is a milled hexagon. The flange is circular, with a diameter of one and one-eighth inch, and a thickness of one-eighth inch. The blank for the thread is seven-eighths inch in diameter, and length not less than one-half inch. The width of the recess adjacent to the flange must not be over the depth of the thread, the latter being seven-eighths of an inch straight in diameter, with 18 inch pitch of the United States standard form and cut perfect into recess. The packing gland nut head is, of course, hexagonal in form and has a short diameter of three-quarters of an inch.
Photo: NYC Mayor George McClellan (November 5, 1908)