tractive effort was at , the latter standard for all Class 25s. Power at rail was , now available between 9.3 and . For the first fifteen locomotives fuel capacity was (design type 25 AV) and the final ten had larger fuel tanks installed (design type 25 BV). Class 25/1 Number sequence (original) D5176–D5232, (TOPS) 25 026–25 082 The Class 25/1 locomotives were built at BR Darlington and Derby locomotive works. They featured the new AEI 253AY traction motor, a result of the collaboration between BTH, MV and American builder Alco. This smaller, lighter motor was an attempt to market a traction motor to a worldwide audience, especially to the metre gauge lines. For Class 25 locomotives these lighter motors meant the discontinuance of other weight saving measures being built into the design. They were highly rated in an attempt to overcome the loss of tractive effort normally found on starting. The field divert system was also modified to allow increased capability throughout all the speed ranges. The main generator was a 12-pole machine with the rating changed to , 780/545V, 1050/1500A at 750 rpm. (The continuous rating has also been quoted as , 630V, 1300A). The four traction motors were now connected as series parallel pairs being rated at , 315V, 650A at 460 rpm, with a gear ratio 18:67. Pairs of motors connected in series provided a higher maximum tractive effort (usually quoted as although could be achieved) but the downside being that a series pair connected machine was more prone to slipping than one with an all parallel grouping. Full power was available between 7 and , an improvement over Class 25/0 locomotives with all other ratings unchanged from the earlier series. The traction motor's continuous rating of 650 amps was not far removed from its one-hour short term or 'emergency' rating of 680 amps, and this could only be monitored manually. On heavy trains close monitoring of the ammeters was necessary to avoid motor damage. Though the body shell remained similar to D5151 there were a number of refinements. The air horns were relocated to either side of the headcode panel. The cab skirt and body fairing were discontinued, though the support lugs remained. A new driving control panel was fitted. The fuel and water tanks were also redesigned with a fuel capacity of (also quoted in sources as being ). There were initially two variants