Fairbanks-Morse renamed their headless Model to "Z" in July 1914, according to engine historian C.H. Wendel. On all "Z" engines the gasoline tank is located in the base. In 1917, they expanded the line to include more sizes. In 1918, they stopped making headless models and adapted the 1.5 H.P. to have a head, and larger H.P. engines could now run on Kerosene. In 1928 The "Z" style "D" was introduced, and was entirely enclosed. The Z was made in incremental sizes of 3, 6, 12, 15 and up to . Over a half million units were produced in the following 30 years. In the early 1980s the line was sold off to Bell. The model Z found favor with farmers, and is a collectable today. During World War I, a large order of 60 30 H.P. CO marine engines were installed in British decoy fishing ships to lure German submarines within range of their 6" naval guns. In 1939 Fairbanks-Morse developed a marine engine using an unusual opposed piston (O. P.) design, similar in arrangement to a series of German Junkers aircraft diesels. The most common variant for submarines through the 1990s was the 38D 8-1/8 engine, ranging from 4 to 12 cylinders. This engine was delivered to the U.S. Navy in large numbers, often for use in fleet submarines, which used 9- or 10-cylinder versions as main engines in World War II. When the innovative but faulty "pancake" engines of the proved unworkable, they were replaced with World War II-style Fairbanks-Morse engines, and these remained standard on US diesel-powered submarines through the early 1960s. These and other Fairbanks-Morse O. P. engines were also used as backup power on US nuclear submarines through the of the 1990s. Fairbanks-Morse ranked 60th among United States corporations in the value of World War II military production contracts. The US Navy has had Fairbanks-Morse diesels in operation on its submarines almost continuously since 1938. They remain in service on -, "Seawolf"-, and nuclear submarines of the US Navy. In addition to O.P. engines (used in the USCGC "Hamilton" class), Fairbanks-Morse license builds Pielstick (used in the s and s), Alco (used in ), and M.A.N. design engines.