The original 14-by-15-inch (356 mm × 381 mm) cylinders were replaced from 1844 by larger 14-by-18-inch (356 mm × 457 mm) or 15-by-18-inch (381 mm × 457 mm) ones.
Reed, P. J. T. (February 1953). White, D. E. (ed.). The Locomotives of the Great Western Railway, Part 2: Broad Gauge. Kenilworth: RCTS. pp. B10–B11. ISBN0-901115-32-0.