A tap-changer for oil-filled electric transformers comprising: - at least one guide element provided with a plurality of electric contacts and at least one slide slidable along said guide element to change the electric connection between two or more of said electric contacts; - moving means of said slide along said guide element comprising a driving group having at least one rotatable portion available to a user and at least one mechanical transmission to transform the rotation of said rotatable portion in a sliding movement of said slide along said guide element, said driving group defining a plurality of discrete positions of said slide with respect to said guide element, each one of said discrete positions corresponding to a different electric connection between said electric contacts, characterised bysaid moving means comprising: - at least one ring-nut susceptible to be put in rotation by said rotatable portion of said driving group and whose side surface has two or more flat surfaces; - at least one core associated with said driving group and counter-moved with said side surface of said ring-nut by first elastic means, the portion of said core faced towards said side surface of said ring-nut having a flat surface susceptible to be arranged one-to-one with the flat surfaces of said side surface of said ring-nut when said slide and said guide element are in correspondence to one of said discrete positions. - at least one guide element provided with a plurality of electric contacts and at least one slide slidable along said guide element to change the electric connection between two or more of said electric contacts; - moving means of said slide along said guide element comprising a driving group having at least one rotatable portion available to a user and at least one mechanical transmission to transform the rotation of said rotatable portion in a sliding movement of said slide along said guide element, said driving group defining a plurality of discrete positions of said slide with respect to said guide element, each one of said discrete positions corresponding to a different electric connection between said electric contacts, characterised bysaid moving means comprising: - at least one ring-nut susceptible to be put in rotation by said rotatable portion of said driving group and whose side surface has two or more flat surfaces; - at least one core associated with said driving group and counter-moved with said side surface of said ring-nut by first elastic means, the portion of said core faced towards said side surface of said ring-nut having a flat surface susceptible to be arranged one-to-one with the flat surfaces of said side surface of said ring-nut when said slide and said guide element are in correspondence to one of said discrete positions. - at least one ring-nut susceptible to be put in rotation by said rotatable portion of said driving group and whose side surface has two or more flat surfaces; - at least one core associated with said driving group and counter-moved with said side surface of said ring-nut by first elastic means, the portion of said core faced towards said side surface of said ring-nut having a flat surface susceptible to be arranged one-to-one with the flat surfaces of said side surface of said ring-nut when said slide and said guide element are in correspondence to one of said discrete positions.