This manual covers Hyundai Mitsubishi S4S and S6S diesel engines, commonly utilized in heavy equipment such as Hyundai Excavator 16 to 18 Ton 7 Series and Hyundai Forklifts HDF35-70 7 Series. These engines are engineered for robust performance and reliability in demanding industrial applications. While specific production years are not detailed, the S4S designation typically refers to a 4-cylinder engine and the S6S to a 6-cylinder engine, both designed to deliver consistent power output. This comprehensive service and repair workshop manual provides essential information for maintaining and troubleshooting these engines. It details general maintenance standards, service procedures, inspections, adjustments, and overhaul instructions. The manual also covers critical engine systems including the fuel, lubrication, cooling, inlet/exhaust, and electrical systems, along with special tools and disassembly preparation. This resource is designed to assist owners, mechanics, and technicians in ensuring the optimal performance and longevity of their Hyundai Mitsubishi S4S and S6S diesel engines.