The Franklin 6A4-150 and 6A4-165 are robust, horizontally opposed, 6-cylinder aircraft piston engines, designed for reliable performance in aviation applications such as Stinson Aircraft. These air-cooled engines, featuring naturally aspirated carbureted induction and magneto ignition, are available in displacements of 232.3 cubic inches (3.81 Liters) for the 6A4-150 model, producing 150 horsepower, and 249.9 cubic inches (4.10 Liters) for the 6A4-165 model, delivering 165 horsepower. Engineered with quality materials like aluminum alloy cylinders with steel sleeves and a forged steel crankshaft, these engines are built for durability and consistent operation. This comprehensive service manual provides essential information for the maintenance and repair of the Franklin 6A4 series engines. It covers a wide range of topics, including detailed tune-up procedures, lubrication specifications, component overhaul instructions, and troubleshooting guidance. By offering in-depth technical data and maintenance schedules, this English-language manual serves as an indispensable resource for owners, mechanics, and technicians to ensure the continued optimal performance and longevity of their aircraft engines.