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Thornycroft Type PC Lorry

Thornycroft’s four-wheel lorry range for 1929 covered the following capacities: 1.5, 2, 3, 3.5, 5 tons and 6 tons (1,524 to 6,096kg). Market conditions required an adjustment to this range, and, for 1930, the 3 and 3.5 tonners (3,048 and 3,556kg) and were replaced by the 2.5 ton (2,540kg) A7 and the 4 ton (4,064kg) Type PC. Two PC models were offered, these being standard and forward-control vehicles. Both models were powered by the four-cylinder L-head HB4 of 5,420cc, uprated to 56bhp at 1,800rpm from its initial 46bhp at 1,500rpm. Full standard equipment included electric lighting, electric horn, air filter, speedometer, spare wheel and tyre and a comprehensive tool kit. Missing from this list, even in 1931, was an electric starter - the driver had to start the engine manually using a starting handle.

The PC followed Thornycroft’s usual lorry design formula. The vehicle had a steel chassis frame supporting the front-mounted, fore-and-aft engine, and power was taken to the back axle from a dry single-plate clutch via a sliding pinion gearbox (i.e. no synchromesh) and a two piece open propeller shaft, delivering power to the differential through overhead worm drive. Metal universal joints supported the external shafting. Disc type wheels were fitted, twins at the rear and singles at the front; pneumatic tyres were standard. Initially, braking was on the rear axle only using a Westinghouse servo system. However the PC was modernised with four-wheel braking in 1931, following the general trend.

In 1931, The Commercial Motor road-tested an example of the forward control PC with a simulated load. The vehicle was taken round a 28 mile (45.1km) course in order to test fuel consumption over a route fairly representing average conditions. The route passed through Worting, Deane, Whitchurch, Bullington Cross, Popham Beacon and back to the Thornycroft works at Basingstoke. Normal cruising speed was about 22mph (35.4kph) and useful maximum was about 30mph (48.3kph). Top gear performance was reported to be good, and the indirect gears only had to be used seven times on the 28 mile (45.1km) test circuit. Gradients of the order of 1 in 14 needed third gear, while 1 in 10 could be tackled in second. First gear coped easily with the steepest hills. It was reported that “In view of the comparative closeness of the third and second ratios it was desirable to change down before the engine speed fell too low”. This could be said of many vehicles, and is probably an observation – almost a statement of the obvious – rather than a muted criticism of the test vehicle. The neat grouping of the instruments in a casing on the steering column was well-received. Average petrol consumption was 9.49mpg (29.7 litres per 100km), about 5.6 per cent above the average for a vehicle in the 4 ton (4,064kg) class. Average speed maintained was 20.2mph (32.5kph).

In keeping with Thornycroft’s new policy of naming its vehicles, while keeping alphanumeric designations, the PC was given the name Sturdy in 1931. The type was still in production in 1932, the final year covered by this narrative.


Specification of Thornycroft Type “PC” Forward Control Chassis (Petrol) August 1931

To carry a net load of 4 tons (4,064kg), with a body allowance of 18cwt (914kg).

Type "HB4" four-cylinder petrol engine, bore
4.375ins (ll1mm) stroke 5.5ins (140mm) will develop 56bhp at 1,800rpm and maximum torque of 228lb-ft (309.2N-m) at 600rpm. RAC rating 30.6hp. Compression ratio is 4.85 to 1 and firing order is 1 3 4 2. The four cylinders are of the monobloc type with detachable head. This facilitates easy inspection of both inlet and exhaust valves, which are on the near side of the cylinder block, side-by-side, and operated from one camshaft. The valve tappets are adjustable and totally enclosed by removable covers; the tappets are of the roller type eliminating any possibility of undue wear. The cylinder block is bolted to the crankcase which carries the crankshaft and camshaft, the crankshaft of large diameter being carried in three long die-cast white-metal bearings, the caps of which are bolted to the top half of the crankcase. The bottom half of the crankcase can be removed without disturbing the main bearings, which ensures an easy means of inspection of the main and big end bearings without removing engine from chassis. The big-end bearings are gunmetal shells with white-metal linings.

Lubrication of engine
The oil pump is contained in the base chamber and immersed in the oil; it is driven by skew-gearing from the camshaft and a large gauze filter is fitted on the suction side. The filtering surface is so arranged that any dirt or carbon falls freely to the bottom of the case and therefore does not tend to choke the gauze which can be removed and replaced through an inspection door on the side of the crankcase. When the base-chamber is removed the pump, drive and filter is very easily removed in one unit by undoing one nut. From the pump, the filtered oil is forced under pressure to a tunnel cast in the side of the crank chamber. From this tunnel there are passages to the three main bearings and the camshaft bearings; the crankshaft being drilled, the oil passes from the main bearings to the big end bearings, which are consequently also lubricated under pressure. A certain amount escapes from the main and big end bearings in the form of a spray and lubricates the cylinder walls, tappets and gudgeon pins, gear wheels, etc. A very large oil filler cap is provided. No copper pipes are incorporated in the lubrication system, except the external pipe to the pressure indicator.

High-tension automatic control magneto. The magneto is mounted on a platform on the near side of the crankcase, and driven by a shaft from the timing case. To enable the timing to be easily set, the flywheel is distinctly marked with the top dead centre position, these markings being set to a fixed pointer.

Float-feed automatic type mounted on the side of the cylinder block; a pilot jet is fitted for starting and slow running. Controlled by foot accelerator with adjustable stop for slow running. An air filter is fitted.

Cooling system
By pump and fan mounted on the front of the cylinder block. The pump is of the propeller type and when out of action does not impede thermo-syphon cooling.

This is of the vertical tube type.

Petrol tank
Mounted externally, to the left hand side of the frame.

A dry single plate 12ins (30.5cm) clutch. A stop allows simple and speedy gear changes.

Mounted on the rear end of the engine crankcase ensuring perfect alignment. Has four forward speeds and a reverse, the top gear being direct drive.

The gear ratios:

  • First 5.28 to 1.
  • Second 2.62 to 1.
  • Third 1.58 to 1.
  • Fourth 1 to 1.
  • Reverse 8.11 to l.

The speed range in top gear is from 6mph (9.7kph) to 30mph (48.2kph).

The drive is taken from the gearbox through an intermediate propeller shaft, and thence to the rear axle through a secondary shaft. The propeller shaft centre bearing is carried in a cross-member attached to the chassis frame.

Back axle
Power from the propeller shaft passes to an overhead worm gear in the differential which drives fully-floating drive shafts. The worm and differential can be removed without jacking up the wheels. The standard differential reduction is 7.25 to 1, and 6.25 to 1 is optional.

Pedal operated brakes operate through a Westinghouse vacuum servo-assistance system. The standard vehicle has brakes operating on the rear wheels only, but brakes on all four wheels are optional. Brakes comprise expanding shoes and drums. On model with four-wheel braking 18ins (45.7cm) drums are used. The hand brake lever expands separate shoes in the rear-wheel drums.

Steering gear
Externally-mounted, giving good access. Allows 3½ turns lock-to-lock and a turning circle of 50ft 7ins (15.4m) right and 53ft 4ins (16.3m) left.

Channel-section pressed steel

Front and rear axles are supported by longitudinal leaf springs. Springs are approximately flat under load.

Wheels and tyres

Disc wheels with pneumatic tyres, 36ins x 7ins (914mm x 178mm), singles front and twins rear.

Wheelbase. 14ft (4.27m).

General measurements and weights:

  • Overall length. 21ft 4.5625ins (6.52m).
  • Body space behind cab 16ft (4.88m).
  • Frame height (laden). 2ft 6.25ins (0.77m)
  • Frame width. 3ft 1ins (0.94m)

Standard equipment includes a speedometer, air filter and generous tool kit.

Electrical equipment
A lighting set and electric horn are provided. A dynamo is fitted to maintain battery charge.