A synchronizer for mechanical transmissions having a phasing system configured so as to include a plurality of first cavities on an internal cylindrical surface of a rim of a hub, the hub torsionally connected with a transmission shaft, forming the external teeth of the hub and, for each of the first cavities, either a tooth which radially protrudes from an external cylindrical surface of a synchronizing ring of the synchronizer and is arranged to engage in the first cavity, or a connecting pin arranged to engage on one side the first cavity and on the other a respective second cavity on the external cylindrical surface, wherein the first cavities have an arc- or round-shaped cross-section.