Case parts of an adjustment mechanism for a seat adjustment device of a motor vehicle can be joined together along a parting line via joint surfaces. The joint surfaces are provided for the purpose of reciprocal placement of the case parts by way of a joining process. Contact surfaces of the case parts are present in addition to the joint surfaces The contact surfaces are in non-parallel alignment to the joint surfaces. Transferred gear forces running parallel to at least a section of the joint surfaces are absorbed by the contact surfaces. The case parts are fastened to one another by way of screw connections. The contact surfaces of the one case part are respectively held in position with respect to the contact surfaces of the other case part solely due to the screw connections. With such a case, high loads can be absorbed in spite of a small size.