Consideration of the boiler began in late 1998. No standard gauge boiler had been built in Britain since the 1960s for such a large express locomotive. It was required to be based on the original LNER Diagram 118 design, but had to meet modern safety standards. Design changes included the cheaper modern-day fabrication method of a welded, rather than riveted, firebox and boiler tube; the use of steel, rather than copper, for the firebox; and the aforementioned height reduction for OLE regulations.