Early in the design process of a copper/brass automotive heat exchanger, such as a radiator or heater core it is desirable to determine the heat transfer performance of the proposed design.Straight forward computer aided design can be used in the form of a BASIC computer program that can be run on a personal computer. By developing such a program an insight can be gained into the mechanisms at work within the heat exchanger core.This paper is a mathematical heat transfer study of a typical copper/brass automotive heat exchanger. The heat transfer mechanisms which are involved in this crossflow type heat exchanger are broken down into their discreet paths. The paths to be studied are the coolant to the tube convective heat transfer, the fin to tube interface, and the fin to air convective heat transfer. From this study an understanding of the critical design features are identified, which is essential for diagnosing which features are responsible for causing a significant reduction of heat exchanger performance especially during production where heat transfer is monitored on a regular basis and a quick and accurate diagnosis is required to correct heat transfer performance. From this analysis, an algorithm wiil be developed for calculating the heat transfer for a given geometry.