British Railways built twenty LMS Stanier Black Fives, numbers 44738-57 with Caprotti valve gear in 1948. Previous black fives had had Walschaerts valve gear as standard. The only previous exception was 1947-built (4)4767 which had Stephenson link motion. (4)4767 and the Caprotti black fives were part of an experimental programme by George Ivatt to try to improve the already good design.
The first of the Caprotti black fives was M4748 which emerged from Crewe in February 1948. It had, in addition to the different valve gear, other modifications which gave it a much different appearance. The boiler was pitched 2 inches higher than previously and the running plate was also lower, necessitating the use of splashers over the wheels. The smokebox was extended and the chimney set further forward. The cab was also a different shape.
M4748 later became 44748 as BR replaced the short-lived M prefix by adding 40000 to most LMS numbers. Numbers 44748-57 were fitted with Timken roller bearings. The last three of these, 44755-7 were also fitted with a double chimney. 44738-47 had plain bearings.
In service, the Caprotti black fives were regarded by railwaymen as faster but less powerful that their Walschaerts valve-geared classmates.
The two ultimate black fives, 44686-7, were built in 1951 with Caprotti valve gear but with more detail variations. No others were so equipped, though construction with various experimental detail differences continued. Later BR built thirty of the closely related standard class 5 with Caprotti valve gear, and it was also employed on the standard class 8 71000 Duke of Gloucester.
None of the Caprotti black fives has survived to preservation, being withdrawn in the 1960s. However, one of the Caprotti standard fives, 73129, and The Duke have both done so.
Many of the successful changes were later used in the design of the BR Standard 5MTs'.