High performance Diesel engines are characterized by remarkable thermo-mechanical loads. Therefore, compared to spark ignition engines, designers are forced to increase component strength in order to avoid failures. By the way, 3D-CFD simulations represent a powerful tool for the evaluation of the engine thermal field and may be used by designers, along with FEM analysis, to prevent thermo-mechanical failures. The current work aims at providing an integrated in-cylinder/CHT methodology for the estimation of a Diesel engine thermal field. On one hand, in-cylinder simulations are fundamental to evaluate not only the global heat transfer at the combustion chamber walls, but also its point-wise distribution. In particular, thanks to an improved heat transfer model based on a modified thermal wall function, wall heat fluxes due to combustion are correctly estimated. On the other hand, a detailed Conjugate Heat Transfer model including both the solid components and the coolant circuit of the engine is adopted, accounting for all the thermo-mechanical effects acting simultaneously during actual operations. Furthermore, a dedicated framework is set up for the piston simulation in order to catch the influence of all the most relevant phenomena such as oil jet impingement and mutual interaction with the surrounding components. The predictive capabilities of the methodology are proved to be effective both in terms of global thermal balance and engine temperature distribution. In fact, numerical coolant heat rejection and thermal field are compared to experimental data provided by thermal survey and point-wise temperature measurements respectively for two different full load operating conditions.