This workshop service repair manual covers the Yanmar BY Series engines, specifically the 4BY and 6BY engine families. The 4BY models are in-line 4-cylinder, turbocharged, water-cooled, 4-cycle diesel engines with a displacement of 1.995L, available in variants such as the 4BY150/150Z and 4BY180/180Z. The 6BY models are in-line 6-cylinder, turbocharged, water-cooled, 4-cycle diesel engines with a displacement of 2.993L, offered in variants like the 6BY220/220Z and 6BY260/260Z. These engines are engineered for robust performance and reliability in demanding applications. This comprehensive manual provides detailed, step-by-step written instructions accompanied by diagrams and pictures to guide users through maintenance and repair procedures. It covers a wide range of topics including general information, torque settings, engine systems, fuel, cooling, lubrication, turbocharger, starter, and alternator systems, as well as electrical and ECU components. Troubleshooting guidance is also included, making it an essential resource for both do-it-yourself enthusiasts and experienced mechanics seeking to maintain and repair their Yanmar BY Series engines effectively.