A torpedo for an extruder comprises: longitudinal member (9) which is provided at the periphery thereof with a plurality of thread portions (12) which protrude outwards and which are spaced apart in the circumferential direction; a plurality of sluice blades (19) which are mounted so as to be movable on the longitudinal member and which are partially engaged in apertures (18) of the longitudinal member, the longitudinal member (9) and the sluice blades (19) being provided with complementary radial guiding surfaces (13, 14, 18) which are in contact with each other; and adjustment means (28) for radially displacing the sluice blades relative to the longitudinal member. The sluice blades extend through the spaces between the thread portions, respectively. The thread portions (12) and the sluice blades (19) are provided with complementary radial guiding surfaces (13, 14, 18).