The 18 hp motors were made for many years, with many minor changes along the way. Not only that, but if I am not mistaken, the 15 you have is a reincarnation of the Gale 15hp, last made in 1963. Bottom line is that comparing parts with older/newer motors, has to be done on a part-by-part basis. See if you can find a specific parts book for your 15 and go from there.