The unusual shape was also dictated by the use of materials; the lagging was made of a glass fibre insulation material known as 'Idaglass', which, although cheap and plentiful during the war years, could not support any weight, and therefore a separate casing was required which followed that seen in the Merchant Navy class locomotives, and the boiler rings were adapted to lend the lagging the support needed. A copper, rather than steel, firebox was utilised, unlike Bulleid's Pacific designs. The wheels were smaller, 5 ft 1 in (1.55 m) adaptations of the Bulleid Firth Brown type utilised on the Pacifics. The locomotive had two cylinders with Stephenson link outside admission piston valves, having a travel in full gear of and a steam lap of . It was provided with a five-nozzle blast-pipe.