Expanding plastic fastener

A plastic fastener, comprising a male member having a head and a pin-like portion having male threads, said pin-like portion depending from the lower surface of said head, and a female member having a flange facing said head of said male member, a leg portion depending from the lower end of said flange and a bore extending from the upper surface of said flange into said leg portion and capable of receiving said pin-like portion of said male portion, said pin-like portion having a diametrically penetrating axial slot which extends in its intermediate portion, said leg portion of said female member having a plurality of axial grooves radially spaced apart around said bore and a plurality of spreadable portions each defined between adjacent ones of said axial grooves and having an upper end connected to the lower surface of said flange and a lower end connected to the lower ends of the other spreadable portions by hinge portions, said spreadable portions being adapted to be spread with the insertion of said pin-like portion into said bore, an upper portion of the inner periphery of said bore being formed with a helical ridge which makes a complete revolution along the inner periphery of said bore and is received in a thread groove defined between the male threads of said pin-like portion, whereby, when said pin-like portion is inserted into said bore, an outer diameter of said pin-like portion slightly contracts, thereby minimizing deformation of the helical ridge by the threads of said pin-like portion and reducing the force necessary to insert said pin-like portion into said bore.