This Service Repair Manual covers Komatsu Hydraulic Excavators, including models PC300-5, PC300LC-5, PC300-5 MIGHTY, PC300LC-5 MIGHTY, PC300HD-5, PC400-5, PC400LC-5, PC400-5 MIGHTY, PC400LC-5 MIGHTY, and PC400HD-5. These machines are engineered for demanding applications such as excavation, construction, and earthmoving. Built with Komatsu's renowned quality and reliability, these hydraulic excavators are designed to deliver robust performance and efficiency in challenging work environments, utilizing engines like the SA6D108-1 for the PC300 series and S6D125-1 for the PC400 series. This comprehensive English-language manual provides detailed guidance for maintenance, assembly, disassembly, servicing, technical maintenance, and troubleshooting. It features detailed illustrations, exploded diagrams, drawings, and photos to facilitate understanding and execution of procedures. The manual is designed to assist owners, mechanics, and technicians in effectively maintaining and repairing their Komatsu excavators, ensuring optimal operational performance and longevity.