A modular, bolt-in, rack and pinion assembly for replacing an existing steering gear box in a motor vehicle. A rack tube, including a pair of elongated slots in its sidewall, houses the rack. The rack tube is mounted to existing holes in the frame of the motor vehicle by means of a plurality of brackets. The rack has a tooth portion and a shaft portion. A pair of studs projects transversely from the shaft portion of the rack, each stud passing through a receptive slot. The studs are mounted to an elongated center link. The vehicle's factory tie rod assemblies are bolted to either end of the center link. A pinion housing is secured over a proximate end of the rack tube, maintaining a pinion gear in threaded engagement with the rack. In a manual embodiment of the invention, an outer end of the pinion is connected directly to the steering wheel shaft. In a powered embodiment, a servo having its own pinion gear is bolted to the pinion housing. An input shaft of the servo is connected to the steering wheel shaft. The servo has high and low pressure supply ports for interconnection to a hydraulic pump, powered by the vehicle's engine. The servo also includes output ports, connected to left hand and right hand stainless steel hard line. The other ends of the hard lines are connected in spaced relation to a distal end of the rack tube. In the powered embodiment, this distal end also houses a power shaft, connected to an end of the rack. A hydraulic plunger, bolted to an outer end of the power shaft, is effective to translate the rack in reciprocating fashion, in response to fluid pressure changes in the hard lines.