This workshop manual covers the YANMAR BY Series marine engines, specifically the 4BY and 6BY models. These engines are engineered for reliability and performance in marine applications, featuring robust construction and efficient operation. The BY Series offers a range of power outputs from 150 HP to 260 HP, with configurations including 4-cylinder and 6-cylinder in-line, turbocharged, 4-cycle diesel engines. Their water-cooled systems ensure optimal operating temperatures, making them suitable for demanding marine environments. This comprehensive workshop manual provides essential information for the maintenance and repair of your YANMAR BY Series engine. It details critical procedures, technical specifications, and troubleshooting guidance for various engine systems, including the fuel, cooling, lubrication, and electrical systems, as well as the turbocharger. The manual is designed to assist owners, technicians, and mechanics in keeping their engines in optimal condition, ensuring longevity and peak performance.