This Service Repair Manual provides comprehensive information for the YANMAR BY Series marine engines, specifically the 4BY and 6BY diesel models. Engineered for reliability and performance, these marine engines are designed to meet the demanding requirements of professional marine applications. This manual is intended to support authorized YANMAR distributors and dealers in their service and repair activities, ensuring optimal operation and longevity of the YANMAR BY Series engines. The purpose of this English-language manual, part number 0BBY0-G00101, is to guide professionals through all aspects of engine maintenance and repair. It covers essential topics including general service information, periodic maintenance schedules, detailed procedures for engine systems such as fuel, cooling, lubrication, turbocharger, starter motor, and alternator, as well as electrical and ECU systems. Furthermore, the manual includes critical troubleshooting procedures to efficiently diagnose and resolve potential issues, ensuring the continued performance and safety of the marine engines.