This workshop service repair manual covers the Yanmar YSM8-R, YSM8-Y, YSM12-R, and YSM12-Y marine diesel engines, designed for reliable marine propulsion. Yanmar is recognized for its engineering quality and the robust construction of its diesel engines. These units are built to deliver consistent performance and durability in demanding marine environments, ensuring efficient operation for a wide range of vessels. This comprehensive English-language manual provides essential information for the maintenance and repair of your Yanmar marine diesel engine. It details general information, engine systems, fuel, lubrication, cooling, starting, and electrical systems, as well as reduction and reversing gears. The manual includes operating instructions, disassembly and reassembly procedures, and inspection and servicing guidelines, all aimed at facilitating routine and preventive maintenance to save time and money by preventing premature failure and unnecessary repairs.