This manual covers the Mitsubishi S4S and S6S Diesel Industrial Engines. These engines are known for their robust construction and reliable performance, making them suitable for demanding applications. The S4S model features a 2.6-liter displacement with 4 cylinders, while the S6S offers a 3.9-liter displacement with 6 cylinders. Both engines are typically naturally aspirated and designed for durability in heavy-duty industrial environments. The purpose of this service repair manual is to provide comprehensive guidance for the maintenance, repair, and troubleshooting of the Mitsubishi S4S and S6S diesel engines. It includes detailed information on engine specifications, lubrication, cooling systems, fuel systems, and electrical systems. Furthermore, the manual offers instructions for general maintenance, overhaul procedures, break-in operations, and performance testing, ensuring that owners and technicians can effectively maintain the engine's optimal performance and longevity.