There were a number of detail variations in the locomotives and they did not all remain in the same condition as built. Some locomotives built under British Railways administration were used as test beds for various design modifications, with a view to incorporating the successful modifications in the Standard Classes of locomotives built from 1951 onwards. These modifications included outside
Caprotti valve gear, roller bearings (both
Timken and
Skefco types) on the coupled and tender axles in varying combinations, and an experimental steel firebox. Other locomotives had modified draughting to "self clean" the smokebox (thereby reducing turn-around and disposal times and eliminating or mitigating one of the most unpopular jobs).
The domeless engines is often mistaken for a
dome Numbering started from
5000, with the first twenty being ordered from
Crewe Works in April 1934, and a further fifty (5020–5069) ordered from the
Vulcan Foundry in 1933. The first of the Vulcan Foundry engines entered service in 1934, and the entire order of 50 was delivered before the first Crewe-built engine, No. 5000, was completed in February 1935. The first 57 locomotives were built with domeless boilers with straight throatplates and a low degree of superheat (14 elements in two rows), the boilers of the remaining 13 (5007–5019) were provided with a three-row version (21 elements) having greater total surface area and giving less obstruction to gas flow. The original 57 boilers were converted later to higher superheat (24 elements) and fitted with a
dome. Further orders were placed with Crewe (5070–5074), Vulcan Foundry (5075–5124) and Armstrong Whitworth (5125–5224) for a total of 155 locomotives which were also built with domeless boilers with straight throatplates and 21 element superheaters. All these boilers, including the early converted ones with a dome, were fitted indiscriminately to any of the first 225 engines, which could appear at various times with domed or domeless boilers. However, many of the early frames were converted to accept sloping throatplate boilers, as listed below. This modification was carried out to provide a stock of spare boilers for the early engines, which would minimise the time spent in works by engines awaiting a fresh boiler. All locomotives from no. 5225 were fitted when new with the sloping throatplate boiler. All extra boilers made had the sloping throatplate arrangement, and only one example of a later engine having been fitted with a straight throatplate boiler is known - no. 45433. Several different patterns of boiler were used on the locomotives, running into double figures. The throatplate design was the most significant, but there were also different numbers of superheater flues, firegrate arrangement, stay material, dome and water feed arrangements, washout plug placement, etc. in various combinations. The following locomotives were built with straight throatplate boilers but were later fitted with a sloping throatplate boiler (date in brackets). Conversion was done by relocating the frame stretcher immediately in front of the firebox. Some of them reverted to straight throatplate at a later date, and these are also shown where known. Those marked with an asterisk were fitted with a boiler which had the top feed on the front ring on the date shown. In the case of No. 45087, it had previously been converted. The first conversion was carried out on no. 5022, and the last known was on no. 45163, which has been preserved. 5002 (12/37), 45007 (1/60), 45008 (1/60*), 45011 (1/49*+), 5020 (2/37), 5022 (10/36) reverted (10/58), 5023 (2/38) reverted (3/53), 5026 (2/37) reverted (1/59), 5027 (12/36), 5040 (11/36), 5045 (11/54), 5047 (1/37), 45049 (7/54) reverted (8/59), 5054 (1/37), 5057 (11/37), 5058 (11/37), 5059 (7/45), 45066 (4/60), 45082 (12/56*), 45087 (9/55) (12/60*), 5097 (1/37), 5108 (6/45), 45109 (5/48), 5142 (12/37), 45151 (3/51), 45163 (5/61), 45169 (7/55), 45197 (5/60)
The pre-war domed engines A further 227 were ordered from
Armstrong Whitworth in 1936, the largest single locomotive order ever given by a British railway to an outside contractor. Crewe built a further 20, which had higher degree
superheat boilers, with 28 elements, unlike the AW boilers, which had 24 elements. 5471, built at Crewe in 1938, would be the last built for five years. During the early stages of the
Second World War, the priority was for heavy freight engines, and the closely related
8Fs were produced in large numbers.
Wartime and postwar domed engines In 1943 construction was restarted, with
Derby Works building its first. Construction continued up to no. 5499. As the numbering block from 5500 was allocated to the
Patriot Class, a further batch of 200 locomotives were numbered from 4800 to 4999, followed by a batch from 4658 to 4799. By this time the LMS had been nationalised, and
British Railways added 40000 to all numbers. Eventually the 842 examples would number 44658–45499.
Ivatt engines and experimental modifications From early 1947, engines were built with the top feed on the front ring of the boiler (from No. 4998), and Nos 44758-767 had a longer wheelbase ( rather than , with the coupled wheelbase extended from to ); this was done to accommodate the Timken roller-bearing housings without fouling the ashpan. In 1948,
George Ivatt introduced more modifications to bearings and valve gear; other experimental Ivatt features included the use of steel rather than copper fireboxes on certain engines, and the fitting of double blastpipes and chimneys in some instances.
44738-57 were built with
Caprotti valve gear. The last two, Nos. 44686 and 44687 built at
Horwich in 1951, were fitted with a new arrangement of Caprotti valve gear, which was later used on some of the BR standard Class fives, and the BR class 8 4-6-2.
No. 4767, built at Crewe and delivered in December 1947, had outside
Stephenson valve gear: instead of eccentrics, double return cranks were used to drive the eccentric rods, and a launch-type expansion link was used. This one cost £13,278, which was about £600 more than those built at the same time with Walschaerts' valve gear. The aim of the experiment was to find out if a valve gear having variable lead (as opposed to the constant lead of the Walschaerts' motion) would affect performance. On trial, it proved to have no advantage, although in normal service it did gain a reputation as a good performer on banks. ==Accidents and incidents==