The need to reduce fuel consumption and exhaust emissions in internal combustion engines has been drastically increased during last years. One of the most important processes affecting these parameters is heat transfer from the in-cylinder gas to the surrounding walls, as this mechanism has a direct influence on the combustion process. Regarding the different walls (liner, cylinder head and piston surfaces), heat flow to the piston is especially important, as it is essential to avoid excessively high temperatures that could result in material damage and/or oil cracking. With this purpose different cooling strategies are used, among which the improvement of the piston cooling system by using oil galleries is preferred. In this work, the heat flow through the oil gallery in a Diesel piston was investigated on a dedicated test bench. This bench consists of a controlled heat source and a piston oil cooling system in which different test conditions were evaluated in order to obtain a correlation for the film coefficient associated with piston oil cooling. These experimental results were then incorporated into a lumped model for engine heat transfer. Finally, in order to evaluate the accuracy of this model and the effects of the correlation for oil gallery coefficient on engine heat flows, results obtained on a conventional engine test bench equipped with a Diesel engine, in which two piston temperatures had been measured, were used. The results show an improvement in piston temperature predictions when compared with those obtained using a previously reported expression for the calculation of the oil film coefficient.