The occurrence of pin boss cracks has become more and more frequent as engine specific power and pcp gets higher on modern engines. The reproduction of these cracks in laboratory is only possible with hydraulic pulsators which allow the presence of oil between the piston pin and piston pin bore. This finding reveals the fact that the the hydrodynamic oil film pressure plays an important role on the piston pin boss crack generation and must be taken into account as boundary condition in any stress analysis which is done on the piston.In this work the piston pin and piston pin boss are simulated as if they were a hydrodynamic sliding bearing. The simulation is done by a computer program that solves the 2D Reynold Equation under dynamic loading. The hydrodynamic pressure field is then used for a 3D FEA of the piston. Experimental measurements were done to confirm the results.