This manual covers the YANMAR BY Series marine diesel engines, including the 4BY2 and 6BY2 models. These engines are designed for marine propulsion and feature a 4-stroke common rail direct injection diesel design, with turbocharged and intercooled aspiration. The 4BY2 model delivers 150 HP, while the 6BY2 offers 200 HP. Engineered for reliability and performance in both saltwater and freshwater marine environments, these engines utilize indirect water cooling and are compatible with various marine drive systems. This Service Repair Manual provides comprehensive guidance for the maintenance, repair, and technical understanding of the YANMAR BY Series engines. It details essential procedures, troubleshooting steps, and technical specifications necessary for optimal engine operation. The manual covers critical areas such as recommended oil types and service intervals, ensuring owners, mechanics, and technicians have the necessary information to maintain these internal combustion engines effectively and safely.