no longer considered necessary. Rotary cam poppet valve gear The rotary cam poppet valve gear consists of two admission valves and two exhaust valves per cylinder, operated by a camshaft arranged in a box fixed to each cylinder between the valve chambers. The cam boxes are self-contained and detachable, and replacement cam boxes can be fitted at running sheds within a few hours. The camshafts are driven from the return cranks on the driving wheels through worm wheels and universal propeller shafts, arranged so that the camshafts and the coupled wheels revolve at the same speed. There were originally eight cams for forward motion and three for reverse, which gave a range of cut-offs from 15% to 85% in forward gear. The hand reversing screw spindle carried a disk, suitably notched with a locking device to ensure that the cam rollers would be in the centre of the selected cams under working conditions. During drifting, the reversing gear is placed in the neutral position where the exhaust valves are held fully open. These conditions were ideal for drifting and superior to any Walschaerts valve gear in admitting steam to the steam chests, even with cylinders equipped with by-pass valves, snifting valves and drifting valves. In service, some trouble was experienced with transmission shaft joints and with worn cams and rollers. If the valves were set with the engine cold, there appeared to be some distortion under working conditions due to the high temperature of superheated steam which caused deterioration of the valve events. Drivers also reported that, when the engines were standing or moving at very slow speed, it was difficult to reverse since the cam rollers were not readily able to climb over the inclined surfaces leading from one cam to another. Trouble was also experienced with steam leaking past the tappet spindles into the cam boxes where it boiled up the oil and caused it to overflow from the cam boxes. This was overcome by serrating the collars of the spindle bushes to allow the steam to escape and prevent entry into the cam boxes. The photograph alongside shows a Class 19C loco­motive after a record speed test run during which the engine achieved a speed of . A.G. Watson is standing sixth from left in the group in front of the locomotive, with hat in hand. An official test was conducted in 1938 to compare the steam