According to an old Champion catalog: 2nd Prefix: R = resistor, or U = auxiliary gap, X = resistor as well 1st Prefix: N = 14mm thread, 3/4"reach, 13/16 hex Basic Number: 1 to 25 = automotive, small engine, ordnance 1st or 2nd Suffix: Y = projected core nose or C = copper-cored center electrode Best Peter