Developing piston assemblies for internal combustion engines faces the conflicting priorities of blow-by, friction, oil consumption and wear. Solving this conflict consists in finding a minimum for all these parameters. This optimization can only be successful if all the effects involved are understood properly. In this paper only blow-by and its associated flow paths for a diesel engine in part load operating mode are part of a detailed numerical investigation. A comparison of the possibilities to do a CFD analysis of this problem should show why the way of modeling described here has been picked. Further, the determination of the complex geometry, which results in a challenging set of calculations, is described. Besides the constraints for temperature and pressure, a meshing method for the creation of a dynamic mesh that is capable of describing the movement of all three rings of the piston ring pack simultaneously is also explained. With these modifications it is possible to calculate dynamic gas flows through the ring pack. For the ring movement it was necessary to coordinate a 1D calculation tool with the CFD analysis. In addition, the ring land pressures and the blow-by have been matched between these tools. The numerical calculation enables velocity profiles, ring land pressures and blow-by graphs to be produced. The averaged calculated blow-by is validated with a measured value of the engine under consideration. The knowledge from this paper should lead to a better understanding of the flow phenomena in the piston ring pack and can be used as a basis for further multiphase calculations containing oil flow.