In 1918, No. 46 was rebuilt by Lawson Billinton with a new boiler including a Robinson superheater. Unfortunately the resulting locomotive was not tested before Billinton decided to rebuild other members of the class, using his K class superheated boiler. The rebuilt locomotives were classified B4X. However, since the original frames could not be used (as the K class firebox was too long to fit between the axles, new frames, new piston valve cylinders), they were virtually new engines. Unfortunately the original motion and motion plate was retained to save costs, which meant that the piston valves were in the constricted space below the cylinders at an angle to the axis of the cylinders. Only 8 inch-diameter valves could be fitted in, and combined with the restricted exhaust arrangement ensured that they were unable to use the steam available from the excellent K class boiler.
