A piston (11) of an automatic machine for infusion of beverages, such as coffee and the like, the piston having a piston head (13a, 13b) divided into two parts connected together and axially movable relative to each other, an annular gasket (15) being housed between those parts and radially expanding when an axially directed thrust such as to make said two piston parts (13a, 13b) approach each other is applied onto said piston (11), wherein the elastic resistance of said gasket (15) keeps said two piston parts (13a, 13b) spaced apart when no axial thrust is applied onto the piston.