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Type PB

This 4 ton lorry was introduced in 1926, with a forward-control chassis designed for use in restricted areas or for delivery work in towns. In common with the normal-control 3 ton (3,048kg) KB described above, the PB was powered by a 5.42 litre HB/4 power unit and was fitted with a transmission brake as well as brakes on the rear wheels. Both the KB and PB had a 13ft (3.96m) wheelbase, and there was little difference in length between both vehicles. The new HB/4 power unit was a side-valve L-head engine and, to that extent was a backward step from Thornycroft's earlier, theoretically more efficient, ioe engines. However, the L-head layout was probably cheaper to build and was much used for motor vehicles for several decades, until overhead valves finally made side valves extinct by the late ''fifties or so. A modern feature of the HB/4 was the gearbox in unit with the engine. Also, electric starting and lighting were options.


Specification of Thornycroft Type "PB" Forward Control Chassis October 1926

To carry a net load of 80cwt (4,064kg), with a body allowance of 18cwt (914kg).


35hp type "HB4" bore 4.375ins (111mm) stroke 5.5ins (140mm) will develop 46bhp at 1,500rpm and up to 50bhp at increased revs. RAC rating 30.6hp. 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 are 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.

A test cock is fitted in the crankcase to indicate the high level point of oil in the sump, and a drain plug is fitted at the lowest point to enable the sump to be drained. An oil pressure gauge is fitted, so that the driver is able to know that the pump is functioning. A dip stick is also fitted, to enable the driver to ascertain the exact level of oil in the crankcase.

Lubrication of Chassis 

Grease-gun system throughout. The system has a quick action bayonet connection and is so designed that the nipple is automatically cleaned when the grease-gun is applied.


Damp-proof 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 positions, these markings being set to a fixed pointer.


Float-feed automatic type, very economical and flexible, with great simplicity; a pilot jet is fitted for starting and slow running. We are constantly experimenting to ensure that the carburettor we fit as standard is the most suitable type for Thornycroft engines. The carburettor is mounted on the side of the cylinder block, and the mixture is warmed by the circulating water and the exhaust. Controlled by foot accelerator with adjustable stop for slow running.

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 gilled tube type, the tubes being 9.5mm bore. The radiator is mounted on cup-shaped rubber buffers, and is built up with removable top and bottom vessels, enabling new tubes to be easily fitted. A drain cock is fitted at the lowest point, so that system can be completely drained.

Petrol Tank

26 gallon (118 litre) steel tank mounted inside the cab, at left of driver's seat.


Single steel plate dry clutch with specially prepared asbestos fabric surfaces, combining simplicity with very light rotating parts, which together with a clutch stop, make the changing of gears very easy.


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. All shafts run in ball-bearings of ample size. Left-hand gear changing is effected by a lever in an invisible gate mounted direct on the gear-box, with no intermediate coupling rods. A speed and mileage recorder, positively driven from the gearbox, is fitted.

The gear ratios are:

First 5.12 to 1. 

Second 2.75 to 1. 

Third 1.56 to 1. 

Fourth 1 to 1. 

Reverse 7.68 to l. 

At 1,500rpm of the engine, with 34ins (864mm) solid tyres and 8¼ rear axle ratio, the vehicle speeds are: 

On 1st speed 4.2mph (6.8kph). 

On 2nd speed 7.8mph (12.6kph). 

On 3rd speed 13.8mph (22.2kph). 

On 4th speed 21.6mph (34.8kph). 

Reverse 2.8mph (4.5kph).


An intermediate propeller shaft connected to the gearbox by a fabric universal joint, the rear end being supported in a ball-bearing flexibly attached to the second cross member. A hollow propeller shaft connects the intermediate shaft to the worm shaft by enclosed metallic universal joints.

Back Axle

Forged steel casing with full-floating differential shafts driven by overhead worm with a reduction of 8¼ to 1. Worm wheel and differential can be removed without removing the axle from the frame.


The hand brake, with control lever fitted on right-hand side of driver, is of the internal expanding types, operating in 22ins (56cm) drums on the rear wheels. The foot brake is of the external contracting band type, operating on a 12ins drum on the intermediate propeller shaft. The brake pads are exceptionally large, ensuring ling life with a minimum of adjustment. Brake pad area per cwt of gross weight is 2.33 ins2 (15cm2).

Steering Gear

Of the screw and nut type, with ball thrust and ball bearings at each end of screw shaft, which renders the steering very easy. This vehicle has a turning circle of 51ft (15.5m).


Channel section pressed steel. The maximum dimensions of the channel are 6.5ins x 3ins x 0.3125ins (16.5cm x 7.62cm x 0.794cm). The construction is such as to give the necessary strength with minimum weight. The power unit is suspended on the three-point system, so that twisting of the frame members due to uneven road surfaces has its harmful effect reduced to a minimum. The front suspension bracket is fitted with a rubber block, to minimise the transmission of vibration to the frame.

Front Axle

The axle body is of "H"-section steel drop forged in one piece. The swivel arms are of nickel steel, with ball thrusts on the pivot pins taking the weight, giving easy steering.


Of spring steel, designed to be approximately flat under normal loading, giving the minimum of movement to the shackles. 

Front 46ins x 2½ins (117cm x 6.4cm). 12 leaves, 1 of 0.375ins (0.95cm), 11 of 0.3125ins (0.793cm).

Rear 54ins x 3ins (137cm x 7.6cm). 12 leaves, 1 of 0.5ins (1.27cm), 11 of 0.4375ins (1.11cm).

Wheels and Tyres

Cast steel wheels with solid rubber tyres, 28ins x 5ins (711mm x 127mm) singles front, 34ins x 5ins (837mm x 127mm) twins rear. All wheels are mounted on taper roller bearings.

Wheel Base

13ft (3.96m).


Front wheels, 5ft 9ins (14.6cm). Rear wheels 5ft 9½ins (14.7cm).

Ground Clearance.

Minimum clearance underneath the chassis fully laden, about 12ins (30.5cm) on standard tyres.

General Measurements and weights

Front axle (laden) 43¼cwt (2.2 tonnes). 

Back axle (laden) 120¾cwt (6.1 tonnes). 

Chassis weight 66cwt (3.4 tonnes). 

Crankshaft diameter 2½ins (6.35cm). 

Gudgeon pin diameter 1.125ins (2.857cm).

Valve diameter 1.875ins (4.763cm). 

Differential shaft diameter 2ins (5.08cm).


An extensive tool kit comprising: 

1 10ins (25.4cm) shifter. 

4 double-ended spanners, 0.1875ins to 0.75ins (4.76mm to 19.05mm). 

1 tommy bar for sparking plug spanner. 

1 sparking plug spanner. 

1 hammer.

1 6ins (15.2cm) screwdriver. 

1 pair of pliers. 

1 magneto spanner. 

1 wheel cap spanner. 

1 tommy bar. 14ins (35.6cm) shifter. 

1 half-link for belt. 

1 oil feeder (1pint (0.568 litre)). 

1 lifting jack. 

1 claw spanner. 

1 tommy bar for claw spanner. 

1 grease-gun. 

1 leather bag for small tools

Electrical Lighting and Starting

Provision is made to supply at an extra charge either a lighting and starting set or a lighting set only, the dynamo being mounted in tandem with the magneto and the starter bolted direct to the engine casing.


A ball type governor, to prevent overspeeding, is fitted.


A thermostat can be fitted to control the temperature of the cooling water.



Chassis diagram 71kb pdf