Between 1956 and 1960, British Railways rebuilt the entire MN class and half of the WC/BB classes to a more conventional design by R. G. Jarvis, adopting many features from the BR Standard locomotives that had been introduced since 1950.[2] The 'air-smoothed' streamlined casing was removed and replaced with conventional boiler cladding, and the chain-driven valve gear was replaced with modified Walschaerts valve gear, whilst the boiler pressure was reduced from 280 to 250psi. The first to be outshopped from Eastleigh was 35018 British India Line in 1956. The final example, 35028 Clan Line, was completed in 1960.
The rebuilt Merchant Navy Class was also to influence the design of the future rebuilding of 60 Light Pacifics. As a result of the rebuilding and the implementation of Walschaerts valve gear, and due to the balance weights on the wheels now required[3] the rebuilds were prone to hammerblow on the track, a complaint that was not evident with as-built locomotives.
The rebuilt locomotives were scrapped relatively soon after their rebuilding, an indication of the waste in resources made by British Railways, as some engines such as 35014 Rotterdam Lloyd were withdrawn as early as 1964.[4] The last few engines in traffic survived until the end of steam on the Southern Region in the summer of 1967.
The rebuilt locomotives were largely successful, though their heavier axle loading of the MN meant that they were banned from certain areas of the Southern Railway, and later, the British Railways (Southern Region) network. The locomotives were often prone to wheelslip, and required very careful driving when starting a heavy train from rest, but once into their stride they were noted for their free running, excellent steam production and being remarkably stable when hauling heavy expresses.[4]
A perennial problem with the unrebuilt locomotives lay with the leaks from the oil bath onto the wheels that in turn splashed oil onto the boiler lagging in service. Once saturated with oil, the lagging attracted coal dust and ash which provided a combustible material, and as a result of the heavy braking of the locomotives, sparks would set the lagging on fire underneath the air-smoothed casing.[5] The local fire brigade would invariably be called to put the fire out, with cold water coming into contact with the hot boiler, causing stress to the castings. Many photographs show an unrebuilt with 'cockled' (warped) casings, a result of a lagging fire.[3]
Another problem was experienced with the soft exhaust, which beat down onto the air-smoothed casing when the engine was on the move, obscuring the driver's vision from the footplate.[6] There was much experimentation in order to resolve this problem, with varying degrees of success, and photographic evidence shows the many guises of this project.[7] The problem was never fully resolved, though the rebuilts were provided with British Railways-style smoke deflectors; the lack of casings on the rebuilds also helped reduce the problem.[8]
An unusual but frequent sight on the 'Withered Arm' (the Southern Region's railways west of Exeter) was of a Light Pacific hauling a local stopping service with a single carriage to destinations as diverse as Padstow and Wadebridge.[4] This highlighted the fact that more of these Pacifics were built than were actually needed, and so could be reduced to undertaking tasks that would usually befit a much smaller locomotive.[4]
BR Rebuilt West Country/Battle of Britain Class[3]
Number 35010 "Blue Star" on Static display awaiting restoration at the Colne Valley Railway
Many of the rebuilt Bulleid Pacifics have survived into preservation thanks to the famous Woodham Brothersscrapyard in Barry, Vale of Glamorgan, South Wales, where steam engines bought from British Railways for scrapping were stored, rather than cut up. One engine, 35028 Clan Line, was bought by the Merchant Navy Locomotive Preservation Society direct from British Railways in 1967.[3] A third of the MN Pacifics have survived into the preservation era, although it is very likely that many will never steam again. Also, no unrebuilt Merchant Navy Class locomotives are in existence.
Image:34050_-_exeter_st_davids_-_1962.jpg|Rebuilt Battle of Britain Class 34050 Royal Observer Corps at Exeter St Davids, 1962.
Image:34056 - exeter st davids - 1962.jpg|Rebuilt Battle of Britain Class 34056 Croydon at Exeter St Davids, 1962.
Image:34108 - exeter st davids - 1962.jpg|Rebuilt West Country Class 34108 Wincanton at Exeter, 1962.
Image:211759 149826e6.jpg|Preserved Rebuilt West Country Class 34028 Eddystone at Swanage, 2004.
Image:SR West Country Class 34045 Ottery St Mary 01.jpg|Rear profile of Rebuilt West Country Class 34027 Taw Valley, masquerading as 34045 Ottery St. Mary at Bridgnorth.
