Boilers
The H and H1 boilers fitted to the "2736" and "3815" classes were larger, having a diameter of 4ft 8in rather than 4ft 1in, and a longer firebox, which made the engines more powerful. While these were being built there started a program of rebuilding many of the earlier engines (but not the first 2 classes) with the "H" boiler to increase their power. By 1915, 380 engines had been so upgraded, giving 450 with "H" and 485 with "B".
Beginning in 1916 engines were rebuilt with Belpaire boilers. Those from the first two classes ("1142" & "1357"), (none of which had received an "H") received the smaller "G6" type boiler (similar size to the "B"), the remainder the larger "G7" size (similar size to the "H"). The "H" & "G7" boilered engines were classed "3" (later "3F") and those with "B" & "G6" boilers were classed "2" (later "2F").
By 1925, production of the new superheated 4F 0-6-0s meant there was no shortage of goods engines of this power class, and from that point only "G6" boilers were installed on rebuilding, sometimes on engines which had previously had "H" boilers, reducing them back to class 2. Three of the later examples were experimentally fitted with superheaters from 1923 to 1928, but generally the class remained saturated throughout. One-hundred thirteen engines remained with their original "B" boilers until scrapped, 22 had "H" boilers, 432 had "G7" and 368 had "G6".
Read more about this topic: Midland Railway Johnson 0-6-0