The instrument has a distance holder (2) for two essentially flat plates (12,13) in which the bearing for a rotator axle (27,28) is mounted. The distance piece holds the plates with the bearing mounted in them and has at least one injection moulded part with an upper and a lower adjuster (3-6). The adjuster enables definite lateral adjustment of the plates relative to each other. The distance holder has stops for the axial adjustment of the plates wrt. each other and wrt. the base of the distance holder.
