A gap adjustment member includes a main body and an outward protrusion. The main body portion has a diameter less than or equal to the diameter of a mounting hole. The outward protrusion protrudes outwardly on an outer perimeter surface of the main body portion, the number of the protrusions being less than or equal to the number of recessed grooves. A diameter of a circumscribing circle, which circumscribes the main body portion and passes a projecting end of the protrusion, and a crossing point, in which a line that passes the projecting end and a center of the main body portion intersects the outer perimeter surface of the main body portion, and which is a crossing point on the opposite side of the projecting end, is configured to be greater than the diameter of the mounting hole and less than or equal to the diameter of an annular groove.