In known stuffer boxes one of the four wall plates enclosing the stuffer box is in the form of a pressure plate which is pivotable about the axis of one of the two rolls which are used to feed the yarn bundle. During operation the pressure plate is pressurised by a pressing force. The yarn material enclosed in the stuffer box exerts a back-pressure on the pressure plate. This results in a force of reaction on the bearing of the roll, which force is dependent on the operational state and the point of application of the pressing force. This leads to fluctuations in the normal force which is exerted on the yarn bundle by the rolls, resulting in uneven crimping. The invention relates to a new device in which the point of application of the pressing force can be moved by an adjustment member (15). A control loop, which comprises a force measurement sensor (17) for the reaction force acting on the roll (2), adjusts the reaction force to zero by actuating the adjustment member (15).