This Service Repair Manual covers the Hyundai Mitsubishi S4S and S6S engines, designed for applications in Hyundai Excavator 16-18 Ton 7 Series and Forklifts HDF35-70 7 Series. These 4-stroke diesel engines are known for their robust engineering and reliability, featuring liquid cooling, direct injection, and forced lubrication systems. The S4S and S6S engines are typically naturally aspirated, with power outputs varying by specific model and application, generally ranging from 50-70 HP for the S4S and 70-100 HP for the S6S. Their construction emphasizes durability and efficiency, making them suitable for demanding industrial environments. This comprehensive manual provides detailed information essential for the maintenance, repair, and overhaul of the S4S and S6S engines. It includes general information, maintenance standards, special tool requirements, and step-by-step instructions for disassembly, reassembly, and inspection. Coverage extends to all major engine systems, including fuel, lubrication, cooling, inlet/exhaust, and electrical systems, supported by highly detailed exploded diagrams and illustrations. This resource is designed to assist owners, mechanics, and technicians in ensuring the optimal performance and longevity of their equipment through accurate and efficient service procedures.