Line-winding spool for grass-cutting heads, comprising a central body provided with a central axis of rotation coinciding with the axis of rotation of the grass-cutting head when inserted into it, said central body having a circumferential side area on which there is defined the annular area where the cutting line is wound, said spool having at least one channel crossing said central body transversally, provided with two openings on opposite portions of said side area for the insertion of the cutting line, said at least one channel having two opposite first segments starting from said openings towards the inside of the central body, and a curved connection segment between said first segments having a concave shape with the concavity upwards, said central body defining, in said concavity, a cavity, whose bottom is at a lower axial height than the axial height of said openings, characterized in that said curved connection segment has a lower surface that is substantially continuous along all its extension so as to be substantially closed downwards, with respect to the outer environment, to allow a single passage of the line from a said opening towards the opposite opening without the line exiting from said central body.
