Abstract: The head comprises two covers (3, 5) connected together and an intermediate ring (7), these defining a volume for containing at least one cutting cord (F). The intermediate ring (7) comprises spring means (7A, 7B, 7C) that lock said intermediate ring (7) angularly to at least one of said covers in such a way as to prevent relative rotation between the intermediate ring and the cover in the direction in which the ring tends to be rotated by the resistive force (F.sub.t) acting on the portions of cutting cord projecting from the head (1) during the rotation of the latter; at the same time the spring means permit relative rotation in the opposite direction by torsionally releasing the intermediate ring from said cover, by the action of the traction exerted on said projecting ends of the cutting cords (F).