The invention describes a starter wherein the second front bearing is inserted axially between the armature windings and the speed reducer. This configuration helps solve the problem that the operating forces of known starter armature shafts are too great for the armature shaft guide ring to withstand long term intensive stresses. The structure includes a supporting section of the armature shaft received in the front bearing, the supporting section being of an outside diameter greater than or equal to the other sections of the armature shaft; the front bearing forms a sealed baffle between the armature windings and the planetary gear speed reducer; the electric motor is housed in a cylindrical frame, coaxial with the armature shaft, the front bearing being centered directly with respect to the frame by radial support against the internal cylindrical face of the front peripheral edge of the frame.