The oil-burning equipment was refitted to Nos. 737 and 739 during the 1926 General Strike and removed in December of that year. Beginning in 1928, all but No. 755 had their cylinder diameter reduced from to when renewals were due, improving speed on flat sections of railway, but affecting their performance on the gradients west of Salisbury. No. 755 The Red Knight was modified in 1940 by Maunsell's successor, Oliver Bulleid with his own design of cylinders and streamlined steam passages. This was married to a Lemaître multiple-jet blastpipe and wide-diameter chimney, allowing the locomotive to produce performances akin to the more powerful Lord Nelson class. Four other N15s were so modified with four more on order, though the latter were cancelled due to wartime shortages of metal. The soft exhaust of the Lemaître multiple-jet blastpipe precipitated an adjustment to the smoke deflectors on three converted locomotives, with the tops angled to the vertical in an attempt to improve air-flow along the boiler cladding. This failed to achieve the desired effect, and the final two modified locomotives retained the Maunsell-style deflectors. The final modifications to the "Urie N15s" involved the conversion of five locomotives (Nos. 740, 745, 748, 749 and 752) to oil-firing in 1946–1947. This was in response to a government scheme to address a post-war coal shortage. The oil tanks were fabricated from welded steel and fitted within the tender coal space. After initial problems with No. 740 Merlin were rectified, the oil-fired locomotives proved good performers on Bournemouth services. A further addition to the oil-fired locomotives was electric headcode and cab lighting, which was retained when the engines reverted to coal-firing in 1948. Performance of the Maunsell batches and modifications The improved front-end layout applied to the first batch of "Eastleigh Arthurs" (Nos. E448–E457) ensured continuous fast running on flat sections of track around London, although their propensity for speed was sometimes compromised over the hilly terrain west of Salisbury. The inside bearings of the Drummond "watercart" tenders proved problematic, as they were too small for the load carried and suffered from water ingress. The retention of the tall Drummond cab prevented use away from the Western section of the Southern Railway. Despite these problems, their operational reliability prompted the management to arrange the visit of No. E449 Sir Torre to the Darlington Railway Centenary celebrations in July 1925. No. E449 also recorded speeds of up to