This workshop service repair manual provides comprehensive information for the YANMAR 4BY / 6BY Series marine diesel engines. These engines, part of the BY Series generation, are known for their robust engineering and reliable performance in marine applications. The manual covers various models within the 4BY and 6BY lines, including turbocharged, water-cooled, 4-cycle diesel configurations with displacements ranging from 1.995L to 2.993L and cylinder counts from 4 to 6. Designed for demanding marine environments, these engines are built to deliver consistent power and efficiency. The purpose of this manual is to serve as an indispensable resource for professional mechanics and DIY users alike. It details essential maintenance procedures, including service schedules and fluid capacities, alongside in-depth troubleshooting guidance. With well-structured contents, high-definition diagrams, and detailed instructions, this manual empowers owners and technicians to effectively maintain and repair their YANMAR engines, ensuring optimal performance and longevity. It provides the same level of detail found in official technician manuals, enabling users to save money on repairs and perform tasks with confidence.