The vehicles were of similar unitary, all-steel construction to the Class 104s. Body framing members, panels & 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 & 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 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 hydraulicaly.

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.

Interior

When built, the first class smoking saloon was in the DMBC, non-smoking in the DMC. Second class passengers who wished to smoke could use either power car, with the non-smoking section being the complete trailer. Over the years, these areas changed to suit the social attitude of the time.

High backed seats were used, as on the 104s, for greater comfort and privacy. Parcel rack vibration was eliminated by the use of rubber gromets between the tubes and mounting brackets. The guard/luggage area had a caged section, with a side corridor for through access.

The interior finishings differed between the two batches. The NER sets had green moquette with a shell pattern in second class, a blue moquette in first, which also had armrests, and carpets, although neither batch of this Class were fitted with curtains. Saloon walls were laminate panelling was pale-green in second and blue in first.

The LMR sets were more lavishly finished, with high-quality veneer panelling in first class. The type of wood was shown on a label mounted above the cab partition doors, and was typically Brazilian zebrano. Second class saloons had the red and pale-green finish which was being used for the new Mark I coaches of the time. Second class seating had green and black upholstery in non-smoking sections and red and black in smoking.

The door windows and sliding door gear came from Beckett Laycock & Watkinson Ltd. Blinds were bought from Nicholls, Fraysse Limited, and extractor ventilators from Greenwoods's & Airvac Ventilating Co Ltd. Sheet plastic was from Formica Ltd, and moquette from John Holdsworth & Co Ltd and JA Wood Ltd. PVC-coated fabrics were by Bernard Wardle Ltd and ICI Ltd. Seat frames came from Cox & Co Ltd and Hallam Sleigh & Cheston Ltd. The latex-foam set fillings were by Dunlopillo Ltd and Bintex Ltd. Carpets were supplied by TF Firth & Sons Ltd, and linoleum from Korkoid Decorative Floors.

Asbestos insulation was supplied by JW Roberts Ltd. Glass came from Pilkington Bros. Ltd.

Screen demisters and washers were from S Smith & Sons Ltd. Warning Horns came from CV Desiderio Ltd, and destination and route indictor gear from EW Matthews & Co.

Problems

In the early days the sets had a few more teething problems than most, which did nothing for their popularity with maintenance staff. The most serious was their habit of catching fire, due to the poor routing of the exhausts and heater problems. The flexible exhaust manifolds, which was required because the Rolls Royce engines had the cylinder heads facing inwards, passed through a hole in one of the inside solebars, travelling along the underside to the rear of the vehicle. This section often worked loose, allowing the exhaust to blow and fill the train with fumes, or cause the whole assembly to fall apart and set fire to the underside of the vehicle. The exhausts were soon re-routed to avoid the solebars.

The heaters were also prone to filling the saloons with fumes, or catch fire caused by unburnt fuel entering the heating ducts. Hammerton St. fitters traced this to an incorrect mixture of fuel and air. The heaters were still under guarantee from Dragonair, and so a modification made which doubled the size of the holes on the heater's burner head to 1/8 in. was strictly against the rules. However, this modification to E51815 was a success and made official and all other vehicles were then altered.

Unfortunately, before the fires were reduced, there were some casualties. A well loaded service train burst into flames in Sowery Bridge Tunnel in 1963. The train was quickly cleared and the passengers managed to escape unhurt, but one power car was not so lucky, being totally destroyed. It was dumped at Sowerby Bridge shed until Hammerton St fitters could strip it for spares before being cut up at the shed. They also cut up 59706 which caught fire at Hammerton Street soon after. The two power cars it was coupled to was saved after the depot foreman uncoupled them, risking his life in the process.

After ten years in traffic, the Class had suffered more fires than any other type. There had been 92 fires in the 60 powers. Numerous modifications had taken place in an attempt to reduce the fire risk, particularly in the area around the exhaust system and heater ducting, where an overspill of diesel fuel from the main fuel filler pipe could saturate lagging around the ducting. These had little affect, and a serious fire on a service train in 1971 at Bescar Lane saw some more drastic action taken.

Refurbishment

Being one of the newest types of sets, refurbishment didn't start until late in the programme. First vehicle done was 51843, outshopped in July 1979, after 2 1/2 years in Doncaster following collision damage. The final one was 52081, exactly four years later, and three months after its partner, 52072.

Alterations

As with most DMUs the refurbishment was carried out in accordance with the BR Director of Industrial Designs Schedule 10. First class seats were reupholstered in a check brown moquette, and the second class in the universal blue/green pattern. Armrests and headrests were re-covered in matching plain vinyl. New orange and ginger carpets were also provided in the first class. First class saloon walls were panelled sand melamine, but second class retained the original pale-green panelling, except if this had been damaged or taken out for asbestos removal. Second class partition walls and doors were resurfaced, or painted blue as appropriate. Fluorescent lighting replaced the tungsten originals. Toilet and guards compartments were also given a facelift, driving cabs repainted and the drivers seat recovered. All surviving vehicles except three trailers were refurbished.

In 1980 Lincoln depot fitted 51827/31 with an experimental hot-water heating system in place of the oil burning heaters. Water taken from the engine cooling system was fed through a single heating unit, and warmed air was passed through the existing Dragonair ducts. 52075/51841 were fitted with four independent water heaters in the saloons, each working independently. 59707 was adapted to have hot water fed from the power cars by flexible pipes on the buffer beam.

These trials couldn't have been too successful, as in 1982 the cars which still had Dragonair heaters went through a replacement programme at Doncaster which fitted Smiths heaters. These had the option if recirculating air from within the saloon, which helped heat it up and involved new air intakes being fitted beneath the seats.

A fire on Class 104 M50440 damaged the roof of Class 110 51823 standing next to it. As the vehicles passed through Doncaster afterwards, much robbing and swapping of parts occurred. However, the only available cab roof was from a Class 104, so the vehicle spent the rest of it's days without a headcode box.