Large low speed hydro-generators have a very small air gap stator bore diameter ratio making it impossible for elements to be perfectly centred during the assembly process. This results in machines operating with a small but not negligible eccentricity due to a large rotating mass which leads to undesirable effects such as considerable unbalanced magnetic pull forces, vibrations and additional losses. It is therefore important to assess eccentricity, minimum and maximum air gaps, stator and rotor shape, the magnetic flux of each pole and, crucially, to check its trend to guarantee safe operation and prevent any serious damage.