A hydraulic unit of a pump for pumping liquid comprising solid matter, the hydraulic unit comprising a housing and an impeller seat, the housing having an axial inlet opening and an outlet opening, and the impeller seat having an axial inlet defined by an inlet wall. The impeller seat has an inlet radius (R) measured from an axially extending centre axis (A) to a circular intersection between the inlet wall and the upper surface of the impeller seat, and the impeller seat comprises a guide pin connected to and extending radially inwards from the inlet wall. The guide pin has a tip radius (r) measured from an axially extending centre axis (A) to the radially innermost part of the guide pin, and has a guide pin angle (μ) defined between a cut water line of a cut water and a radius of the impeller seat.