The paper presents a method for predicting instantaneous heat transfer between the working fluid of an indirect injection diesel engine and the combustion chamber surfaces in main and pre-chamber. The correlation proposed is for both motored and fired engine operation and is supported by experimental evidence.The model is based on the conventional Nusselt-Reynolds number relationship for the convective term, but using a novel approach for the local characteristic velocity term. Under fired conditions, velocity enhancement is included together with a radiation model.An experimental programme of instantaneous heat flux measurement in pre- and main chamber is described, under a wide range of engine operating conditions, both motored and fired. Comparisons of measured and predicted heat fluxes, in both chambers, are given. Also included are mean values of main and pre-chamber surface temperature, and connecting passage wall temperatures.