stronger than the spoked equivalent. These proved to be successful and were later used on other Bulleid classes. The leading bogie was based upon that of the SR Lord Nelson class, although it had a 6 ft 3 in (1.90 m) wheelbase as opposed to Maunsell's 7 ft 6 in (2.28 m) design, and featured 3 ft 1 in (0.94 m) BFB wheels. A long coupled driving wheelbase was incorporated into the design, to keep the locomotives within the lineal loading of the Southern Railway's narrower bridges. The supporting rear trailing truck was a one-piece steel casting that gave the smoothest of rides; the design was utilised in the future BR Standard Class 7. The spaces between driving wheels housed steam-powered clasp brakes, that gripped the wheels by way of a "scissor" action. The two middle brake hangers held two brake blocks each, whilst the two outside hanger on the leading and rear driving wheels held one block each. These were connected together by outside rodding for ease of access, and the whole system was operated from the footplate. Boiler and welded firebox The maximum boiler pressure was higher than any other British regular service locomotive (except the GWR County class) at 280 psi. Bulleid decided on cheaper all-welded fireboxes for the boilers as opposed to more common riveted construction, and a steel inner firebox which was lighter than a more usual copper example. Two welded steel thermic syphons were implemented to improve water circulation around the firebox and these were subcontracted to Beyer Peacock. However he soon discovered that the Southern Railway lacked the facilities to manufacture welded boilers of this size, so the first ten boilers were ordered on outside contract from the North British Locomotive Company. Air-smoothed casing, smokebox and blastpipe The boiler was enveloped by Bulleid's air-smoothed sheet-steel casing, which was not for the purposes of streamlining, as demonstrated by the extremely flat front end, but as a way of lifting exhaust gases. The flat sides were also an aid to cleaning the locomotive with mechanical carriage washers, representing an attempt to reduce labour costs. It followed the profile of the Belpaire firebox and extended to a curved profile forward of the smokebox front. Spun glass mattresses were used for boiler lagging. The smokebox was a sheet metal fabrication to the same profile as the firebox, acting as a former to maintain the shape of the