GEC, which were then known as WT54A motors. The gearing was altered so that cars with either size of wheels remained compatible. Trials with the 'loudaphone' system by which the guard and motorman could communicate on this stock resulted in it being retro-fitted to the earlier stock. During this period, many of the Gate Stock cars on the Central line had been converted to air-door operation by the Union Construction Co., which was based at Feltham. Plans were briefly formulated to convert the Gate Stock on the Piccadilly and Bakerloo lines, but analysis showed that the cost of building a new car was only slightly more than the cost of converting an old one, and so an order for 182 new cars was placed. They were known as the 1927 Feltham Stock. Their door plates carried the date 1928, and they were not delivered until 1929 and 1930. The lightweight construction caused problems in later years, when the seat risers and body bolsters developed fractures. With the batch of cars built by MCWF and fitted with BTH equipment, these vehicles enabled Gate Stock to be withdrawn from the Piccadilly line in June 1929, and from the Bakerloo line on 1 January 1930. 1929 and 1930 builds Another 53 cars were ordered from the Union Construction Co. in 1929, to enable the remaining 1920 Stock cars, which had air doors but were not really suitable for operation on the open sections of the Piccadilly line, to be transferred to the Bakerloo line. An extra train of two motor cars and four trailers were ordered in 1930, to test several new features. They were the last cars to be built by the Union Construction Co., for it was closed as a result of political pressure in 1932. The new motor cars were longer, and the trailers were longer. This was achieved by tapering one or both ends, to prevent them fouling the structure gauge on curves. On two of the trailers, the centre door openings were widened from to . On the other two, the width of the centre doors was not altered, but the seating capacity was reduced from 48 to 40, to allow the provision of an additional single-leaf door at both ends of the car. The position of the guard's door control panel was altered, although this was not found to be satisfactory, and was dropped from subsequent builds.