Abstract: Flow restriction assemblies (10,10a) adapted to be mounted on single or twin screw extruders (17,102) in order to allow selective adjustment of material flow through the extruders (17,102), with consequent alteration in back pressure and shear conditions. The assemblies (10,10a) each have shearlock(s) (12,12a) mounted on the extruder drive shaft(s) (34,34a), together with restriction units (14,14a). The units (14,14a) have opposed flow restriction components (48,50,48a,50a) supported on opposite sides of the shearlock(s) (12,12a) and mounted for a substantially aligned and rectilinear back and forth sliding movement. In this fashion, the clearance between the inner surfaces (54,55,54a,55a) of the components (48,50,48a,50a) can be selectively adjusted.