The vehicles were of similar all-steel construction to the Class 104s. Body framing members, panels and roof were all of 16-gauge mild steel fabricated by electric arc welding into jig-built units riveted together and to the underframe. The underframe, made up of four solebars, was a welded structure of 5" deep rolled-steel channel and plate fabrications forming the bolsters and headstocks. The pan frame, unit type windows were designed by BRCW to accomodate the size of glass used in standard coaches, and to comply with the new BR requirement to avoid panel edge corrosion.
Each power car had 2 Rolls Royce series C6-N FLH 130D engines of 180hp. These were coupled by a cardan shaft to a Self Changing Gears SE4 epicyclic gearbox. Another shaft connected to an AEC P239C final drive on the inner axle of the bogie nearest the engine. It had a ratio of 2.97:1 giving overall ratios of change speed and final drive gears of 12.6:1, 7.15:1, 4.74:1 and 2.97:1. Also engine driven were the fan cooled radiators. A through-floor atmospheric cooling system was used, the header tanks being floor mounted in the passenger saloons behind the seats next to the bulkheads at the end of the centre saloon to avoid inconvenience to passengers by loss of leg room.
Power for control, lighting and battery charging on the power cars came from dual engine driven CAV AC 8 alternators, with a combined maximum output of 130 amps paralleled through a single CAV RUG11 rectifier and regulator unit. On the centre cars, the first batch had an axle driven Stones Tonum AR30LC generator (with a maximum output of 150amp), and the second batch had CAV AC 14 alternators. Both power and centre cars had a battery of 12 Chloride Exide KFD33 lead acid cells with a capacity of 430 amp/hour at the five hour rate.
The standard BUT control system used CAV type relays for the first time in operation of throttle, change speed and reversing gears, oil and air pressure indication. The main underframe wiring was carried in sheet metal trunking and conduit branches were kept to a minimum to enable the cable loom to be made up on the shop floor. Terminal boxes were fitted to both ends of each vehicle, and in the middle of the power cars for connections to the power and control equipment.
Graviner automatic fire protection equipment was fitted to protect angines and gearboxes. Flexible connections were provided by Lockheed Precision Products Ltd. Fuel Tank gauges were from Bayham Ltd, and air-system pipe fittings from The British Ermeto Corporation Limited.
In each vehicle a single Dragonair DLVA80 independent oil-fired heater supplied saloon heating at 80,000 BTUs per hour through ducts along each side. Ozonair filters were used, and the whole of the underframe hot-air ducting was moulded in glass-fibre laminate lagged with mineral wool and protected with plasticised glass-cloth covers.
Wheels and axles were from Owen & Dyson Limited, and axleboxes came from Timken. Bolster coil springs were supplied by Wilford & Co Ltd and Thomas Turton & Sons Ltd. The side-bearing laminated springs came from Steel Peech & Tozer Ltd, and Woodhead-Monroe Ltd supplied the hydraulic dampers. The self-contained buffers were by George Turton Platt & Co. Ltd. Rubber springs were bought from Aeon Products Ltd, JH Fenner & Co Ltd and George Spencer, Moulton & Co Ltd. Non-metalic liners were from British Belting & Asbestos Co Ltd.
Bogies were the Derby design, with hydraulic dampers on the power cars to control lateral movement rather then the usual leaf springs. The centre cars bogies were modified in accordance with a redesigned BTC suspension layout, which included vertically-hung swing rods carrying on knife edges the spring plank, which was tied to the bolster by an "anti-shimmy" bar; side control was by rubber springing and hydraulic dampers; and secondary springing, also hydraulically damped, which included a rubber spring of progressive effort. This took increasing affect with the addition of payload, this reducing the equivalent rate, and this portion of the suspension was also damped hydraulically.
After testing BRC&W claimed that the trailers were remarkably stable, even at 50mph, on track with a ruling gradient of 1 in 80 and with reverse curvature of 30 chain radius, whilst on sections laid out for fast running no apparent deterioration in riding characteristics occurred up to speed of 75 mph. The second batch of sets were slightly modified with a light metal frame rather than a wooden support plank.
The standard Gresham and Craven twin-pipe vacuum brake system was used. At one time all vehicles carried shed plates on the solebar (on both sides) showing their home depot.centre cars.
By 1985 51823 had been fitted with a Class 104 cab roof dome.