As a means of increasing the tractive effort to bring it closer to the requested by Pole, smaller 6 ft 6 in (1.981 m) driving wheels were used compared to the standard 6 ft 8.5 in (2.045 m) on the "Castles" and the first six locomotives to be built had their cylinders bored out to giving a further , thereby enabling the ‘Kings’ to achieve a tractive effort of . The smaller wheels also allowed for a wider boiler within the loading gauge to be used.