A method for building a magnetic core including laminations bound into packs for an electrical machine comprises steps of: stacking a group of laminations one on top of the other to build the core; binding into a pack the group building the core by locking members that axially compress the group with a predetermined clamping force; pre-compressing, before clamping the pack by the locking members, the group by a pre-compression press that is independent and separate from the locking members; measuring an axial length of the group while the group is being compressed by the pre-compression press; initially forming the group with a normally lacking number of laminations to initially have the measured length that is lower than or substantially equal to a desired length; and adding, after having completed pre-compression, an additional number of laminations to the group determined according to a difference between the desired and measured lengths.