Abstract: An adapter which comprises a pre-manufactured assembly capable of infinite vertical adjustment and self-adjusting width capability. To this end, the adapter of the present invention comprises a two-part unit in which the two parts thereof are relatively shiftable widthwise of the rack, yet a first of the parts is telescoped within the second part and secured by screws to the adjacent side of the chassis unit to be mounted. The second part of the adapter can be snap-fitted into the central, hollow portion of a vertical rail of the rack and normally remains stationary with respect to such rail; whereas, the first part of the adapter can be inserted into the second part before the second part is inserted into the hollow center portion of the rail.