Large low speed hydro-generators (Fig. 1) have a very small specific air-gap to stator bore diameter ratio making it virtually impossible to have a perfect centering of the rotor within the stator during the assembly process. Therefore the machines are operated with an eccentricity that, though small, is not negligible, and is the cause of undesirable effects: considerable unbalanced magnetic pulls, vibrations, additional losses. It is therefore important to assess the eccentricity and even more to check its trend in case of any stator or rotor deformations in order to guarantee a safe operation and to prevent any serious damage at an early stage.