Pre-war, the Marles steering box was fitted to the Bentley M-series chassis, the last few 20/25 and the 25/30 chassis, Wraith and Phantom 3.

The Marles steering is designed to have no free play within the steering box when the steering is in the straight ahead position, and within a range of thirty degrees either side of the central position of the steering wheel. Outside this range and on either lock there is significant free play within the box by design; however, on the road this free play is not apparent because of the load on the steering.

If free play develops within the box when the steering is in the cental position it cannot be eliminated by adjustment. The free play arises due to the wear of internal components and bearings, and can be overcome only by replacing the worn components.