<p>The piston is one of the most essential parts of an internal combustion engine. Direct adjustment of the piston-cylinder clearance, i.e., oil-film thickness between the piston skirt and the cylinder liner, in designs, repairs, and engine conversions, as well as indirectly through mode selection, lubrication, and cooling regimes in engine operation, are necessary to improve the efficiency of its operation. Directly measuring the piston-cylinder clearance in the firing engine is extremely difficult, requiring complex equipment and test benches, but it is difficult. The article presents the theoretical basis and results of converting the original piston of the YAMZ-236 engine to a piston with an oval-barrel profile, using the finite element method in ANSYS 17.2 software to calculate the interaction of the original piston-cylinder pair and the piston-cylinder pair with the modified piston to determine the piston-cylinder clearance, as well as strike out about the effect of using oval- barrel pistons on a particular engine. In the firing engine, the thermal gap of the original piston crown with the cylinder has a negative value, indicating that the original piston-cylinder pair has no gap at the piston crown. The oval-barrel profile of the piston improved the lubrication, reducing the risk of jamming, and reducing noise, friction.</p>