Abstract: The impactor includes a grinding face 4 disposed underneath the impact aprons 3 at the side of the housing 1 opposite the feed material inlet, the grinding face partially extending under the rotor 2 and connected with a hinged rear housing wall 5 so that it executes a movement away from the rotor when the rear housing wall swings outwards. According to the invention, the grinding face 4 movably rests on a base member 15, a rail or plate, attached to the housing 1, and the lower part of the grinding face 4 is pivotally connected to the rear housing wall 5 by a first linking element 11 at a pivot point 18 located above the pivot element 6 of the rear housing wall 5, and the upper part of the grinding face 4 is pivotably connected to the rear housing wall 5 by a second linking element 12 at a pivot point 19 located a greater distance away from the pivot element 6 than pivot point 18.