This workshop repair service manual covers YANMAR Marine Diesel Engines designed for marine propulsion. These engines are vertical, 4-cycle, water-cooled diesel units with varying displacements and cylinder counts, including models like the 1GM10, 2GM20, 3GM30, and 3HM35. Built with YANMAR's reputation for engineering quality and reliability, these engines are a robust choice for maritime applications. This manual provides comprehensive guidance for owners, mechanics, and technicians. It details repairs, maintenance, and servicing procedures, covering all essential systems such as the fuel, lubrication, cooling, and electrical systems, as well as the reduction gear and reversing gear. With detailed exploded views, step-by-step written procedures, pictures, and diagrams, this resource ensures proper upkeep and efficient troubleshooting for your YANMAR marine diesel engine.