Construction
The general construction is based on an integral design with an all-steel underframe: the body was constructed entirely of aluminium alloy extrusions and panelling and completely integrated with the underframe thus affording maximum stiffness. Sound and thermal insulation was provided by filling the space between the interior and exterior panels throughout with a lightweight plastic insulating material. Acoustic panels, two inch thick, were fitted below the floor, which was covered throughout with sound deadening material below the wearing surface. All the windows were safety glass glazed by the Eastern Coach Works Fastflex system. Six hopper type ventilating windows were fitted to each side of the body. The glass was supplied by Pilkington Bros Ltd. Droplight windows were supplied by Beckett, Laycock & Watkinson Ltd.

BR standard headcode lamps were fitted, supplied by BMAC Ltd.. No buffers were fitted, but two tubular units, at standard buffer height and width, directly connected with diagonal struts to the main longitudinal frame members, supported a bumper frame.

Power
The single Gardner 6HLW 6-cylinder engine was of 8.4 litres capacity, delivering 112bhp at 1,700rpm, and was flexibly mounted below the body floor at the centre of the vehicles. To obviate the measurement of the oil level by dipstick, an engine oil dispenser was fitted which automatically topped up the oil sump from a reservoir tank. A sight glass gauge, visible from the outside of the vehicle, showed when the reservoir tank required filling. To ensure a clean air supply to the engine the air intake was taken from the roof by ducting.

A Coventry radiator, of flattened tube type, was suspended below the vehicle floor between the engine and trailing axle. The header tank and filler cap were at the side of the vehicle and accessible through a suitable aperture in the skirt panel. The coolant capacity of the system was about 10 gallons. A single 38 gallon fuel tank was mounted below floor level at the end on the vehicle and supplied both the engine and the heater.

The battery was an Exide type 3 BMF 17/2 of 296 A-hr capacity, this was charged by a CAV ac generator type 824/2 via a rectifier and 24V dc control board. The alternator was driven by a vee-belt from the front end of the gearbox.

The hydraulic auxiliary throttle control was supplied by Automative Products Co Ltd.

Transmission
The engine was fitted with an 18 inch Self Changing Gears Ltd fluid flywheel unit which incorporated a centrifugal lock-up clutch to provide a positive drive at speeds above 700rpm. The drive from clutch to gearbox was through a short cardan shaft embodying a free-wheel unit. The Self Changing Gears Ltd five speed (overdrive) epicyclic R11B gearbox was arranged for fully automatic gear selection and engagement by the SCG Ltd VS automatic control system. (a Derby lightweight set was trialled with this). Gear engagement was by air pressure, controlled by Westinghouse electro-pneumatic valves operating in response to signals from the VS automatic control system. The control system was evolved by Bristol Commercial Vehicles in collaboration with Westinghouse and Self Changing Gears. Westinghouse pneumatic equipment was used for throttle and brake controls and for gearbox operation. Transmission control was fully automatic from standstill to maximum speed.

The drive continued from the gearbox via another cardan shaft to the double reduction final drive unit. The single driving axle was a Kirkstall pot-type steel forging. It was similar to a bus or heavy lorry axle, except that it had no differential. There were two spiral bevels for forward and reverse, which were engaged by dogs operating from air cylinders. An interlocking device prevented any movement of the vehicle until the dogs were correctly aligned. A manually operated neutral position allowed the vehicle to be towed if necessary. The wheels on the trailing axle could revolve independently.

Suspension
The wheels were of the Svenska Atkibolaget Bromsregulator (Sweden) resilient type, the rims were flexibly mounted relative to the wheel centres by a system of rubber bushes, reducing wheel shock and noise when traversing rail joints. The wheels were detachable from the hubs.

Suspension was by Metalastik rubber units in two stages. Each axle was attached to a sub-frame through two pairs of Chevron-type rubber sandwiches; the main structure was supported from the subframe by Metalastik rubber spring units, four at each subframe. In side elevation the axles of the spring units made an angle of 15 degrees with the horizontal and the load was taken by the rubber in compression and shear. Damping was by separate vertical and horizontal telescopic hydraulic units (supplied by Girling Ltd) and brake torque reaction links were fitted at both axles.

Brakes
The Dunlop Monitor brake system was arranged for pneumatic/hydraulic operation of discs at each wheel. A beam carrying a Monitor shoe was pivoted on two vertical links, which in turn pivoted about their mounting brackets. An air operated cylinder was positioned in one vertical link which brought the monitor shoe into contact with the wheel rim, thus causing the vertical links and beam to swing in the direction of wheel rotation. An extension arm from the second vertical link energised a hydraulic double-acting master cylinder, which applied fluid under pressure to the caliper operating cylinder, forcing the friction pads into contact with the disc. Air supply to the brake operating cylinder was controlled manually by a lever on the drivers desk. Each brake was independent although all brakes were supplied together. Hydraulic fluid was supplied from two tanks, each tank having separate compartments for each brake.

A model of the Dunlop monitor brake

Oil level in each tank was shown by a sight tube. Vents for air cylinder and caliper cylinder were piped through transparent plastic hoses to points above possible flood water level. Vent pipes extremities were fitted with filters. The brakes could be mechanically operated by means of the handbrake lever in the drivers cab. Pad clearances with brakes off were automatically maintained at a constant amount - no adjustment was necessary. Pads were changed when fully worn by removal of a split pin and nut, allowing the carrier plate to be withdrawn. The caliper arms were then opened by a tool provided and the new pads secured in position. The monitor shoe was retained by a pin and a split cotter; bearing pin bushes were of the oil-less type and did not require lubrication. Hydraulic fluid levels had to be maintained within the sight level aperture. Bleeding of air was from a single point provided on the upper side of each caliper cylinder. As the wheel rims were kept clean by the rubbing of the monitor shoe on the rim, good contact for track-circuiting was made between the wheel and rail.

The bus-type mechanical hand brake operated on all four wheels and was sufficiently powerful, being assisted by the automatic servo action of the Monitor brake system, to allow the vehicle to continue to operate should the air brake fail.

Interior
The interior was finished in cream leather cloth with green moquette seating, facing forward in each saloon in a 2+3 layout,and an enamelled white ceiling. The moquette was supplied by T. C. Firth (1955) Ltd. The drivers compartment interior was finished in dark green leathercloth. Parcel racks were fitted on each side of the passenger compartments and were of plastic netting and alloy tube. They also carried a stainless steel handrail and were hung on polished aluminium alloy brackets. Saloon lighting equipment was supplied by Lewis Dixon Ltd & EX Matthew's Ltd.

Centrally placed on each side of the body were single power-operated sliding doors, under the control of the driver. These had a safety interlock to ensure that the vehicles could not be driven until the doors were closed. Sliding door pneumatic gear and the seat frames were supplied by Deans & Son (Yorkshire) Ltd. A Smiths oil burning heater supplied warm air to each passenger and driver compartment via ducting.

Cab Driving controls were reduced to the minimum and made very simple in action. Only a single removable master control key was used. The controls consisted of an accelerator lever incorporating the deadmans handle, air brake lever, horn lever, hand brake lever, and direction lever for forward and reverse motion selection and engine stopping and starting (the master removable key). It was claimed that the whole driving technique could be mastered within about ten minutes. The handbrake was of the bus type and mechanical, operating on all wheels and arranged for independent application or release from either end of the vehicle. Indicating instruments mounted on the driver's control panel included, an engine cooling water thermostat, air pressure gauge and speedometer and mileage recorder. Windscreen wipers were of the Trico FPK heavy duty type. The drivers seat was supplied by AW Chapman Ltd.

Dimensions