Snapping unit for heads of harvesting machines, including a pair of counter-rotating snapping rolls and a support and motion transmitting unit with which the snapping rolls are associated so as to cooperate with each other when placed into rotation around their longitudinal axis, each snapping roll including a body where a cylindrical rear portion and a frustoconical front portion are defined, the side wall of the body being provided on its outer surface with longitudinal helical grooves defining helical protrusions ending in corresponding helical ridges capable of penetrating the groove defined between the helical ridges of the adjacent roll during rotation of the two snapping rolls, wherein the helical grooves and, correspondingly, the helical protrusions and the respective helical ridges have a pitch at least equal to the total length of the corresponding roll and extend on at least part of the frustoconical portion.
