First Three Vehicles
The bodies were designed by BUT but manufactured for them by Park Royal, and the underframe and mechanics by AEC. Both were ACV companies. Initially they were numbered Cars 1-3, but later given the BR numbers M79740-2. As a 3-car set the length was 120ft 9ins over buffers, weighing 39tons 4cwt.
They showed an average fuel consumption of 11½ to 12mpg per car, or 5¾ to 6mpg for a 2/3 car set,
Further Eight Vehicles
These were built in two batches, 79743-7 in 1955, as part of a £4m order placed with BUT, mainly for power trains for DMUs, and 79748-50 in 1957
These eight cars didn't have the
bodysides skirts, and had sliding lights rather than droplights on the
sides.
Power Train
Each power car was driven by a 125hp BUT engine developing a maximum speed of 1,800rpm and driving through a 4-speed epicyclic gearbox. The reverse gear was incorporated in the double reduction final drive mounted on the axle between wheels and a freewheel was incorporated in the propeller shaft between the engine and gearbox.
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Cabs The power cars had a cab at both ends, allowing them to be used singly.
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Tyres
By August 1956 it was found that the 3-car set in use on the St Albans - Watford line wore out steel tyres at five times the rate of the tyres used on the bogie wheels of DMUs. There was also damage caused to the more delicate parts of the motor by the hammering at rail joints. Trials with pneumatic tyres were abandoned, due to the tremendous wear on the rubber tyres.
Interior
Seats were provided for 129 in the first three
cars, as the plans on this page show. The further eight vehicles would
carry 110 passengers (in a 3-car)
and there was considerable standing room. In these one power car had seats for 34, the second just
28 but included a separate luggage comp.
A combustion heater was fitted in each car providing warm air through outlet grilles at each seat position.
The seats were bus-type and upholstered in a strawberry pink moquette on at least the first 3-car set. A single passenger access door was provided either side in the centre of the body.
Modifications
By November 1957 the exhaust pipes had been modified, now being discharged at roof rather than rail level, although it was thought the piping, brought up vertically on one side of the central windscreen creating a very ugly effect.