Five members of the class were fitted with a double chimney at different times. 5684 "Jutland" was the first, fitted with a double Kylchap in 1937. The double chimney did improve the power of the locos and also improved the coal consumption. It only carried this fitment for one year. 5742 "Connaught" was the next, being fitted with a plain double exhaust in 1940 which it carried until 1955. 5553 "Canada" was also fitted in 1940 but carried the double chimney for a short time. 5735 "Comet" and 5736 "Phoenix" were rebuilt with a 2A taper boiler and double chimney in 1942. They were to have been the prototypes for the rebuilding of the entire class but were, in the end, the only Jubilees so to be treated. (They were reclassified 6P in July 1943, and 7P in 1951). As part of experiments at the Rugby Locomotive Testing Station, no. 45722 "Defence" was fitted with a double chimney from 1956 to 1957. In 1961 a double exhaust was fitted to no. 45596 "Bahamas" which carried it through withdrawal and into preservation.