Peter Hunn’s "The Old Outboard Book" lists a page from Champion from 1954 (Page 298 in the third addition) that shows the D model Elto Light Twin (AKA Rudder Twin) using a 7 Champion spark plug. Then on page 300 it gives an update on Champion spark plug numbers. There it wrongly equates the 7 and C7 to a D16. The Champion C7 is a hotter plug than the 7. The latest number for the 7 is Champion 516. The latest number for the C7 is Champion 526. These given numbers are educated guesses by the factory and actual plugs used will depend on load, temperature, humidity. and alditude. At one time the quality of Champion spark plugs made them the standard of the industry. Outsourcing to reduce cost has made this no longer true. Good luck on finding the proper spark plug . . . 😎
