Between the two half-molds (2, 3) fixed against each other in order to form an envelope (1), one-piece blow-molded air tubes (12a, 12b) are mounted; the outlet (14a, 14b) of the tubes is held in a half-mold (3) by means of a fixing clamp (6) to the cylinder-head of the engine while being sealingly connected to an opening (7) of the clamp (6) for supplying one cylinder by means of a tubular nozzle formed of a cradle (20a, 20b) molded with the clamp (6) and the half-mold (3), and a staple (22a, 22b) fixed to the cradle to hold a ring (17) on the outlet of the tube while compressing a seal (18) between the tubes (12a, 12b) and the nozzle. Application particularly to intake manifolds of engines with multipoint injection and controlled ignition.