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Abstract: The paper deals with the problems related to the development of a new piston design for replacement during repair in 2-stroke D-100 engines for locomotives. The paper addresses the design, the mass of which is 35% less than the mass of the prototype. The results of thermal calculation, which served the initial data for the temperature and heat transfer coefficients evaluation over the heat-receiving surface of the combustion chamber of the piston, coincide with the experiment of prof. G. Rosenblit. The integral boundary conditions were refined by calculating five successive engine cycles, α_P = 1830 W / m²K and T_{eq p} = 970 K were selected for further research. Next, heat transfer from the piston internal cavity to the oil, which is supplied through a nozzle in the small head of the connecting rod, was studied. The study included a 3D simulation of the cooling oil movement from the nozzle to the inner surface, both in the stationary mode (Earth's gravity field) and taking into account the variable acceleration of the piston at the design mode. In this case, the location of the holes in the nozzle, the flow parameters and the properties of the oil were simulated. The study showed that taking piston acceleration into account leads to a slight difference in heat transfer coefficient in the center of the piston (830 - 800 W / m²K) and rather large at the periphery (600 ... 520 W / m²K taking into account the acceleration of the piston against 460 ... 420 W / m²K). It is shown that the nozzle with 4 holes and the angle of inclination of the side holes φ = 55º gives a more efficient cooling.