This workshop service and repair manual covers the Yanmar 1GM10(C), 2GM20(F)(C), 3GM30(F)(C), and 3HM35(F)(C) marine diesel engines, designed for reliable marine propulsion. These diesel engines are engineered for durability and performance in demanding marine environments, featuring robust construction and efficient operation. The manual provides comprehensive information for maintaining these engines to Yanmar's high standards. This manual is an essential resource for owners, technicians, and mechanics. It details critical maintenance procedures, troubleshooting guides, and technical specifications for the engine's general operation, basic engine components, fuel system, governor, intake and exhaust system, lubrication system, direct sea-water cooling system, fresh water cooling system, and reduction and reversing gear. By following the instructions within this manual, users can ensure the optimal performance and longevity of their Yanmar marine diesel engines.