In mid 1980, Caterpillar started production of the 3500 direct injection diesel engine family. Initial ratings were 74.6 kw (100 hp) per cylinder at 1800 rpm and 61.9 kw (83 hp) at 1200 rpm. To provide added value to the customer, meet competitive pressure, and provide desired power for Caterpillar captive vehicles, Caterpillar initiated the Phase 1 uprating program. This program stressed increasing specific engine output and reducing specific fuel consumption. The prime strategy involved in the Phase 1 uprating program was to improve combustion through higher peak cylinder pressure and faster rate of heat release during the combustion process.The purpose of this paper is to describe the engine design changes that were required to accommodate the increased structural and thermal loading encountered in the Phase 1 uprating program. The required fuel system changes are not included here and are expected to be the subject of a later technical paper.