The aim of this work of thesis is the presentation of the steps followed during the application of World Class Manufacturing for the improvement of the assembly process. The project has been designed in FPT Industrial S.P.A. plant in Turin. As a consequence of a repeated issue reported by the customers, the decision to improve the assembly process, in order to avoid the re-occurrence of the problem. In specific, the issue was the leakage of oil due to the wrong pitching of the front crankshaft seal. To carry out this study and design activity have been exploited the tools given by World Class Manufacturing, helpful to define the weaknesses of the assembly process and find a solution.