Billinton's final contribution was to order the rebuilding of 12 B4 4-4-0 locomotives, which were fitted with K class type boilers and new frames. Only two were completed by the LBSCR in 1922, the remainder being converted by the Southern Railway. Although the B4x class (as it was designated) now had larger and more modern superheated boilers the rebuilds retained the valve gear of the originals, with the result that the locomotives were throttled by small short-travel valves which could not pass the steam needed for an improved performance. By 1930 the class were relegated from main line duties.