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Different numerical-experimental methods have recently been introduced in literature for the diagnosis of structural damages by using location-dependent changes in modal parameters (natural frequencies, damping factors, mode shapes, etc.). The main task is the determination of presence, location and extent of the damage. This work is concerned with the possibility of introducing, along with a manufacturing process, an on-line quality control for structural components prior to assembly using modal parameters. Based on the natural variations of different components, the problem of error measurements in connection with available modal data could become more noticeable than the health monitoring of a particular system. Indeed, the modal data that should be used in diagnosing routines belong to different systems that are obtained by diverse molding cycles carried out over various metal sheets. For this reason a deviation of the modal data, with respect to a mean value, should be assessed to appreciate the possibility of introducing an on-line quality control. Moreover, experimental work benchmarking the deviation of such modal data in the case we are dealing with is not available. In this work a certain number of structural components are experimentally tested in order to appreciate the deviation of the modal data. With this in mind, comparisons among mode shapes, natural frequencies and damping factors have been carefully examined and their relevant usability is discussed.