The mounting wrench (2) is used to engage or disengage a tool head (24) of a modular tool. The mounting wrench (2) incorporates a base body (4), which extends from an upper surface (20) along a centered longitudinal axis (18) to an underside (22). In addition, it has a slotted part (6), extending in the direction of the centered longitudinal axis (18), with a slot (10), into which the tool head (24) can be inserted. The slotted part (6) has two opposing keys (14), extending radially inward, which are each designed to mesh into a keyway (30) of the tool head (24). The keys (14) have a rounded end face (32), oriented radially inward, and are sloped relative to the centered longitudinal axis (18) so that the distance (a) between the keys (14) increases in the direction of the centered longitudinal axis (18).