This workshop service repair manual covers engines from the JCB make, specifically the 4HK1-6HK1 Isuzu engine families, designed for application in various JCB equipment. These diesel engines feature a common rail direct injection fuel system and are engineered to meet evolving emission standards. The 4HK1 is a 4-cylinder engine with a 5.2 L displacement, while the 6HK1 is a 6-cylinder engine with a 7.8 L displacement, both sharing a 114 mm bore and 122 mm stroke. Their robust construction and advanced fuel systems contribute to their reputation for reliability and performance in demanding operational environments. This comprehensive manual provides essential information for the maintenance, repair, and troubleshooting of these engines. It details recommended engine oil grades and capacities for both the 4HK1 and 6HK1, as well as coolant type and general maintenance procedures for filters. The manual also addresses common issues such as engine starting problems, performance and emission system faults, and cooling system concerns, indicating the necessity of specific diagnostic tools like the ISUZU G-IDSS for effective troubleshooting. This English-language resource is designed to assist owners, mechanics, and technicians in ensuring the optimal operation and longevity of their JCB equipment.