The Komatsu 5 Series excavators, including models such as the 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, represent a robust line of heavy machinery engineered for demanding applications. These machines are built with a focus on reliability and performance, incorporating advanced diesel engine technology with models like the SA6D108-1 for the PC300 series and the S6D125-1 for the PC400 series. Their construction is designed to withstand challenging work environments, making them suitable for a wide range of construction and earthmoving tasks. This Workshop Service Repair Manual provides comprehensive technical information for the Komatsu 5 Series excavators. It covers essential areas including general information, structure and function, testing and adjusting procedures, disassembly and assembly, and maintenance standards. The manual is designed to assist owners, mechanics, and technicians in performing routine maintenance, troubleshooting issues, and executing repairs with high-quality diagrams and instructions. Its content is structured to empower users, including Do-It-Yourselfers, to save money on service and maintenance while ensuring the longevity and optimal performance of their equipment.