Unusually if not uniquely, the 5AT will be fitted with twin valves above each cylinder combined with large volume steam chests. Valves must be sized to ensure that the total port area through which the steam flows into and out of the cylinders, is sufficient to keep pressure drops to an acceptable level.
Twin valves offer a significant advantage in that their combined mass is much lower than that of a single valve offering the same port area. Thus inertial forces at very high speed are reduced even though these are small compared to those caused by the pistons and drive motion.
The use of twin valves also facilitates the provision of a large steam chest volume, essential for maintaining high steam pressure during the admission phase of each piston stroke. Conversely their provision has the negative effect of increasing clearance volume.
Use of twin valves necessitates use of a combination lever that is suspended from a rocking lever. The suspended combination lever changes the geometry of the standard Walschaerts valve gear layout, and therefore slightly affects the valve motion that is derived from the valve gear. Dr Allan Wallace kindly undertook the task of modifying his valve gear simulation software program to assist David Wardale in verifying his predictions of valve events on the 5AT.