A shell stator (12, 14) for a vacuum pump, the shell stator (12, 14) comprising: a seal groove (50, 52) formed in a surface of the shell stator (12, 14), the seal groove (50, 52) being defined by a first side wall (60), a second side wall (62) opposite to the first side wall (60), and a bottom surface (64) disposed between the first side wall (60) and the second side wall (62), the bottom surface (64) being opposite to an opening of the seal groove (50, 52); wherein the seal groove (50, 52) comprises: a protrusion (68) formed in the first side wall (60) and extending towards the second side wall (62); and an indentation (69) formed in the second side wall (62) opposite to the protrusion (68).