This study deals with a numerical procedure designed and built to evaluate the evolution of soot deposit thickness and their impact on heat transfer for a high-pressure common rail 16V280 marine diesel engine piston. Using a combination of 3D numerical computations and an iterative calculation algorithm, this work reveals the complex relationship between soot deposit, temperature distribution, and piston thermal dynamics. Non-uniform soot deposit distribution is observed, concentrated at the piston bowl peripheral regions. This distribution aligns with theoretical expectations, indicating the influence of the swirl effect. The presence of soot deposits alters the temperature distribution, implying displacement of high-temperature zones towards the bowl region of the piston. The reduction in surface temperature of approximately 14% is attributed to the lower heat transfer coefficient of deposited layers. This greatly influences the distribution of thermo-mechanical stresses of the piston. The proposed procedure offers an approach to assess the impact of soot deposit on heat transfer. In addition, this study contributes to a better understanding of realistic piston conditions and addresses the challenges introduced by soot deposits in heavy-duty diesel engines by combining the proposed procedure with investigations based on CFD software tools.