This manual covers the Yanmar 4BY2 & 6BY2 marine diesel engines, part of the BY2 Unlimited Series. These engines are engineered for reliable operation and performance in marine applications. Their advanced electronic control interface ensures efficient power delivery and responsiveness. This documentation provides essential information for understanding and utilizing these robust marine diesel engines. The purpose of this manual is to serve as a comprehensive guide for the operation, installation, and maintenance of the Yanmar 4BY2 & 6BY2 marine diesel engines. It includes detailed sections on safety, product overview, pre-operation checks, engine operation, periodic maintenance, long-term storage, troubleshooting, and technical specifications. With step-by-step instructions and detailed illustrations, this manual equips owners and technicians with the knowledge necessary to maintain the optimal performance and longevity of their Yanmar engines.