The results of comparative design research in the thermal stress and strain state of the piston in transport diesel engine 2 F 10.5/12 are given for its rated power operation. The standard piston is made of an aluminium alloy, and the modernized one, of steel. Piston thermal profiling and indicator test results were used for identifying the mathematical models and refining the boundary conditions for mechanics and heat conduction problems. To ensure reliable heat rejection from the piston, the paper considered the case of oil jet cooling. This was taken into account when describing the boundary conditions of the heat conduction problem. The thin-wall steel piston with oil jet cooling was shown to function reliably under the study conditions. The temperature in the first compression ring groove does not exceed 200 °С, and the radial deformation of the piston crown is less than half of that of an aluminium alloy standard piston.