As originally built the class had a Swindon superheater with an area of . However, in 1947 experiments were undertaken with a four-row high-degree superheater in No. 6022 "King Edward III". As a result, the four-row superheaters were fitted to the whole class, and modifications were also made to the draughting arrangement, using No. 6001 "King Edward VII" as a test-bed. From September 1955, double blast-pipes and chimneys were fitted, initially to No. 6015 "King Richard III". Following successful testing the whole of the class was subsequently modified and, as a result, their final years in British Railways ownership saw the very best of their performance, particularly on the steep South Devon Banks at Dainton, Rattery, and Hemerdon.