A cleaning tool (1) for use in cleaning ferrous material from a wellbore has a magnetic cleaning element (14) mounted to a tool body (2) within a slot (5) using a releasable concealed interference fit fastener assembly (15) comprising a fastener member having a head (16), and a shank (17) with a configured end (18) and a deformable fastener ring (20) adapted to fit closely over the shank, and a press-fit collar (21) adapted to deform the deformable fastener ring upon the configured end of the shank when assembled, and wherein the respective head and collar are each flanged to permit an interference fit with a corresponding contact surface of the tool body around said slot.