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Type KB

The 2.5 ton (2,540kg) BX was replaced in 1926 by the 2 ton (2,032kg) A2 and 3 ton (3,048kg) KB models. The KB was introduced initially as a 2.5 ton (2,540kg) lorry according to Thornycroft's April 1926 specification for this vehicle, but The Commercial Motor of 7 December reports that "the new KB model is now to be described as a 3-tonner (3,048kg)". The KB had the new 5.42 litre HB/4 power unit comprising engine and gearbox in one unit, mainly to minimise production and operating costs, but unit design also offered compactness and improved alignment between engine, clutch and gearbox. The Type KB's HB/4 engine was Thornycroft's second unitary design for its lorries, the first being the smaller FB/4 used in the A1, A2, A1RSW and A3RSW. Introduced in 1926, the KB's straight 'four' HB/4 developed 46hp at 1,500rpm corresponding to 22.1mph (35.6kph) with the KB's gearing, although the engine was said to be good for 55hp at unspecified rpm. In an attempt to save weight, Thornycroft used duralumin connecting rods in the HB/4, but problems with these resulted in a reversion to forged steel conrods. The KB had newly designed cast wheels with eight webbed spokes.

Having been omitted from certain models, the transmission brake re-appeared on the KB. However transmission brakes would, eventually, become things of the past on vehicles. Extras included electric lighting and starting, and a gearbox-driven tyre pump for use when pneumatic tyres were fitted. Careful driving of the KB under full load returned around 9mpg (31.4 litres/100km).

The Commercial Motor journal carried out a test drive in November 1926, taking a short route a few miles from Basingstoke, which included Farleigh Hill. The journal reported that the KB maintained a "highly creditable turn of speed" and took the hill easily. Another vehicle of apparently the same load capacity completed the long approach to the hill, but entered the real rise with its radiator already boiling, and was brought to a standstill on the steepest portion. The Commercial Motor reported that the KB was pleasantly quiet and that the gears changed easily. The journal also reported that the brakes were good and the power ample, and the KB was praised for the ease with which it pulled away again from the steepest part of the hill after an intentional stop.


Specification of Thornycroft Type "KB" Chassis April 1926

To carry a net load of 50cwt (2,540kg), with a body allowance of 20cwt (1,016kg).


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 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.

A test cock is fitted in the crankcase to indicate the high level point of oil in crankcase, and a drain plug is fitted at the lowest point to enable the sump to be cleared out. 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 in applying the grease-gun, the nipple is self cleaned.


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 position, 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 5/16 ins (7.94mm) bore, and mounted on cups fitted with 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

14½ gallon (65.9 litre) tank carried on the dash providing gravity feed to carburettor. The tank is fitted with a device which retains the last 2.5 gallons (11.4 litres) until the position of the plug of a two-way cock is changed. This prevents the tank being run empty without warning.


Single steel plate dry clutch engaging with specially prepared asbestos fabric surfaces, combining simplicity with very light rotating parts, which together with a clutch stop makes 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. Gear changing is effected by a central lever in an invisible gate mounted immediately above the gear-box. 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 36ins (914mm) solid tyres and 7.25 back axle ratio, the vehicle speeds are: 

On 1st speed 4.3mph (6.9kph). 

On 2nd speed 8mph (12.9kph). 

On 3rd speed 14.2mph (22.8kph). 

On 4th speed 22.1mph (35.6kph). 

Reverse 2.9mph (4.7kph).


An intermediate propeller shaft connected with the gearbox by a fabric universal joint, the other end 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 7.25 to 1. Worm wheel and differential can be removed without removing the axle from the frame.


Rear wheel and transmission brakes are fitted and are of the internal expanding shoe type operating in 20ins (50.8cm) drums for the rear wheels with control lever on right-hand side of driver and of external contracting band type operating on a 12ins (30.5cm) drum for the transmission brake on the intermediate propeller shaft. The Ferodo pads are exceptionally large, ensuring long life with a minimum of adjustment.

Brake pad areas are as follows:-

Rear wheels 243ins2 (1,567cm2) total on 20ins (50.8cm) drums. 

Transmission 89ins2 (574cm2) total on 12ins (30.5) drum.

Steering Gear

Of the screw and nut type, fitted outside the frame with ball thrust and ball bearings at each end of screw shaft, which render the steering very easy. The vehicle can be turned without reversing in a road 45ft (13.7m) wide.


Channel section pressed steel. The maximum dimensions of the channel are 7ins x 4ins x ¼ins (17.8cm x 10.2cm x 0.635cm). 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 taper roller thrusts on the pivot pins, giving easy steering, the track rod being behind the axle body.


Of silicon-manganese steel and exceptional length, giving very easy riding. Designed to be approximately flat under normal load, giving the minimum of movement to the shackles. Rear springs are attached to the axle by our patented holding-down bolt system, which prevents the stretching of the bolts, and the top leaves are designed with ears which bear against the spring palm brackets, thus ensuring that the axle is held in true alignment with the frame without the disadvantages of radius rods.

Front springs 48ins x 3ins x 0.625ins (122cm x 7.6cm x 1.59cm). 

Rear springs 60ins x 4ins x 0.625ins (152cm x 10.2cm x 1.59cm).

Wheels and Tyres

Cast steel wheels, bolted to cast steel hubs, which run on taper roller bearings for both axles. This allows the wheels to be removed without disturbing the roller bearings to change the tyres. 36ins (914mm) for 30ins x 5ins (762mm x 127mm) solid tyres are fitted, singles front, twins rear. (Alternative tyres available).

Front Mudguards 

Are of ample dimensions.

Wheelbase. 13ft (3.96m).


On 36ins x 5ins (914mm x 127mm) solid tyres. Front 74½ins (189cm). Rear 71 ins (180cm)

Ground clearance

A large clearance, i.e. 10ins (25.4cm) approximately.

General Measurements and Weights

Front axle (laden) 39¾cwt (2.0 tonnes). 

Back axle (laden) 79¾cwt (4.1 tonnes). 

Chassis weight 54½cwt (2.8 tonnes).

Crankshaft diameter 2½ins (6.35cm).

Gudgeon pin diameter 1.125ins (2.856cm). 

Valve diameter 1.875ins (4.763cm). 

Differential shaft diameter 2ins (5.08cm).


2 side lamp brackets. 

1 tail lamp bracket. 

1 14ins (35.6cm) shifter 

4 double ended spanners 0.1875ins (0.476cm) to 0.75ins (1.905cm).

1 tommy bar 52703. 

1 sparking plug spanner 52707.

1 hammer 633. 

1 6ins (15.2cm) screwdriver. 

1 pair of pliers. 

1 magneto spanner. 

1 tommy bar 628. 

1 half-pint oil feeder 637. 

1 lifting jack. 

1 4ins (10.2cm) shifter. 

1 spanner for rear axle plug. 

1 ½-link for belt. 

1 grease-gun. 

1 instruction book. 

1 spanner for rear axle nut and plug.


A speedometer or mileometer positively driven from the gearbox is fitted

Electric 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.

Tyre Pump

A mechanically operated pump can be supplied at an extra charge and provision is made for driving this off the side of the gear-box, the pump being bolted direct to the gear-box.



Chassis diagram 98kb pdf