Blow molded plastic bottle and plastic cap

A molded plastic cap, for use with a blow molded plastic bottle having a neck, said neck having a mouth, an external screw thread below said mouth and an inwardly directed portion at said mouth, said plastic cap adapted to be screwed onto said neck to cover said mouth and having an upper wall, a cylindrical side wall with an internal thread corresponding to said external thread for engaging said external thread, and a downwardly directed circular plug for engaging said inwardly directed portion, when said cap is screwed onto said neck, and to form an antileak seal with said inwardly directed portion, said cap having antitamper means for cooperating with means on said bottle and including a permanently deformable element, said antitamper means permitting said cap to be rotated in one direction to screw said cap onto said neck to bring said plug into sealing relation to said mouth and then preventing rotation in an unscrewing direction unless said element is visibly deformed, said cap having upwardly and inwardly extending means at the inside of said cap above said internal thread for providing a space to receive the top of said neck and for restraining the top of said neck against substantial outward movement so as to keep said mouth in good sealing contact with said plug, said cap being molded of a polyolefin plastic, having an axial height of about 1/2 inch and an external diameter of about 11/2 inches, the walls of said cap being so thin that said cap weighs less than about 21/2 grams, said threads being double-start threads comprising a pair of external threads of identical pitch having effective portions substantially disposed around the periphery of said neck and a pair of internal threads of identical pitch having effective portions substantially symmetrically disposed around the internal periphery of said cap.