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Types A1 and A2

Introduced in 1924, Thornycroft's 1.5 ton (1,524kg) A1 lorry complied with the requirements of the War Office's subsidy scheme, whereby each user who joined the scheme received an annual subsidy of £40 per vehicle for three years. In return, the user would sell the lorry to the War Office, on demand, in the event of a national emergency. The subsidy could apply to the chassis or the complete vehicle, depending on whether or not the body met War Office subsidy requirements. If the chassis only was subsidised, then the body would be removed and retained by the user in the event of purchase by the War Office.

The A1 was available both as a standard or subsidy lorry. Users not wanting to join the subsidy scheme could buy the standard A1 chassis stripped of certain fittings called for by the War Office, and thus reduce the first cost of the vehicle. Extra fittings to the standard A1 which made it eligible for the War Office subsidy comprised 34 x 7ins (864 x 178mm) pneumatic tyres, a mechanical tyre pump (driven from a power take-off on the gearbox), a 12volt 5amp lighting set, lamp irons, a suitable freight body and a dash-mounted chassis lubrication plate engraved with a lubrication diagram. The latter was specified because, during the 'twenties (and later), the need for regular greasing of chassis wearing parts was normal on vehicles; this was carried out by applying a grease gun to grease nipples located at various points on the chassis.

This Type A1 petrol tanker is displayed at Milestones. Photo: Nick Corrie

The new A1 was pleasing to the eye, particularly with pneumatic tyres, thanks to its sloping, tapering bonnet, supported by a hint of style from the wings which had a slight reverse sweep at their trailing edges. Also, when finished in the house colours of its owner, the A1 could look magnificent. The A1's standard Thornycroft cab looked like that of a contemporary steam locomotive, but, unlike some steam locos, the A1 had no glass in its side "windows" - just draughty cutaways!

The A1 was powered by the relatively small FB/4 monobloc straight four of 3,620cc, with side valves, a three-bearing crankshaft, aluminium pistons and forced oil feed. A first for Thornycroft lorry power units was the fact that the A1's engine and gearbox formed a single unit, a feature which was then becoming normal practice in vehicles and is now virtually universal. During that generally pre-diesel era for road vehicles, the L-head FB/4 was petrol driven and gave 36bhp at 1,500rpm - about the same output, incidentally, as an 850cc Mini car of 35 years later, although at much lower rpm. Thornycroft advised that the FB/4 was good for up to 40bhp at increased, but unspecified rpm. The A1's transmission comprised a four-speed gear box and rear-wheel drive, normal on all Thornycroft lorries, except the 2 ton (2,032kg) BT Type which had only three-speeds. Power was transmitted through a propeller shaft, universally jointed at each end with fabric joints, to a worm-driven back axle. True to its time, the A1 had (drum) brakes on the rear wheels only. However, there was no transmission brake because, once a normal fitting on Thornycroft vehicles, its popularity was diminishing.

The standard Thornycroft fuel tank location, on the dash, was specified for subsidy lorries. This location had the advantage of eliminating the need for a petrol feed pump. Non-subsidy owners could, if they wished, locate the petrol tank elsewhere, e.g. at the rear of the chassis. In the latter case petrol would have been supplied to the carburettor, typically, by an Autovac pump rather than by gravity. The Autovac was vacuum operated from the inlet manifold, and was much used on road vehicles at one time.

For non-subsidy A1s, Thornycroft could supply at extra charge a tyre inflator for £10 driven from the gearbox (standard on subsidy A1s), a lighting set for £23 (standard on subsidy A1s) or a dynamo-energised lighting set and starter motor (£36). Direct current dynamos eventually became normal fittings on vehicles until superseded by alternators during the 'seventies.

Without Thornycroft's starter option, the A1's engine was swung into action by hand using a starting handle engaging with a sprocket on a forward extension of the crankshaft. The starting handle could be seen protruding forwards from beneath the radiator. Indeed, the starting handle was almost a badge of office on lorries at one time, a reminder that some poor soul might have to hand swing a large, heavy and possibly bad-tempered engine into life. Even when electric starters became standard fitments on vehicles, starting handles were still provided for many years afterwards, and they were useful in the event of a failed starter or a poorly-charged battery. However, when vehicle electrics became more reliable, manufacturers eventually stopped fitting emergency starting handles to their products.

Dunlop pneumatic tyres were offered for the A1, but solid tyres could be fitted. In 1925, Thornycroft offered nine alternative standard freight bodies for the A1, namely three van body options and six types of lorry body (plain lorry platform, brewer's lorry, open-sided lorry, hinged-sided lorry, end-tipping lorry and War Department body). However, the end tipping lorry body was not fitted to subsidy vehicles, and the WD body was not fitted to standard vehicles. In 1926, Thornycroft added a 350 gallon (1,591 litres) watering wagon body to its standard options. Customers could also have special bodies fitted to suit their own requirements, and Shell, for instance, had six A1s fitted out as petrol tankers (see photograph).

From Thornycroft's June 1926 price list, the bare A1 chassis cost £460 with 34 x 7ins (864 x 178mm) pneumatic tyre options, and standard Thornycroft body prices varied between £60 for the simple open lorry body, to £116 for the more complex watering wagon. Cab side doors and glass windscreens were included in the price, except for the WD body, and Thornycroft made something of the "very full equipment" supplied as standard with the A1 chassis, which included "Speed and Mile recorder, Radiator guard, etc". There was no heating or air conditioning, just open voids for side-windows in Thornycroft's standard non-WD cabs, and there were no windscreen wipers. From our heated, air-conditioned, fully instrumented and cosseted environment of today, one can only admire the hardy lorry drivers of the 'twenties, many of whom had to spend hours in cold, drafty cabs with no creature comforts.

The A1's gearing gave it a speed of 25mph (40kph) at its 1,500rpm "normal speed" (as Thornycroft called it). With the lower traffic density and reduced speeds of the day, Thornycroft evidently considered that brakes on the rear wheels only were effective enough. Thornycroft estimated a maximum of 37mph (60kph) on the level, at which speed the engine would have been turning at 2,220rpm, possibly past its peak power speed. Had this figure been accurate, then the A1 would have been Thornycroft's fastest lorry when it was introduced. However, on test of an A1 in 1924, The Commercial Motor journal witnessed a test in which the speedometer showed 30mph (48kph) on the level (1,800rpm).

The A1's chassis weight of 1.65 tons (1,677kg) was little more than half that of the J, and the A1 was regarded as a light lorry with its 1.5 ton (1,524kg) load capacity, although this was (and is) not evident to people near an A1, above whom the lorry towered majestically! The wheelbase was 11ft 6ins (3.51m), but the A1 long chassis option was introduced in 1925 with a 14ft (4.27m) wheelbase for passenger work only. Long-distance petrol consumption of the A1 lorry was a reasonable 18mpg (15.7 litres per 100km) on normal road conditions and oil consumption was 800mpg (0.35 litres per 100km), which seems outrageously thirsty nowadays.

The A1 was a successful chassis and many buyers were not looking to the subsidy to justify buying one. As well as for lorries, the A1 chassis was used for motor-coaches and small buses (20-seats), and by late 1925, Thornycroft had supplied many hundred A1 bus and lorry chassis for service at home and abroad. On the home front, A1 sales contributed to the 300,000 plus commercial vehicles in the UK at that time. For the year ending 31 July 1925, Thornycroft's company report records that during the year the firm's commercial vehicle sales increased by over 50 per cent, and there are frequent mentions of orders for Thornycroft vehicles in contemporary editions of The Commercial Motor. The journal said that by mid-1926, the majority of orders for Thornycroft vehicles were for the 1.5 ton (1,524kg) A1 chassis. Bodies of exported A1s were sometimes built overseas.


Specification of Thornycroft War Department Subsidy Type "A1" Chassis December 1924

To carry a net load of 30cwt (1,524kg), with a body allowance of 10cwt (508kg), and suitable for 20-seated omnibus and coach bodies.


25hp type "FB4", bore 3.75ins (95mm) x 5ins (127mm), will develop 36bhp at 1,500rpm and up to 40bhp at increased revs. RAC rating 22.5hp. 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; exhaust and inlet valves are interchangeable. The valve tappets are adjustable and totally enclosed by removable covers; the tappets have exceptionally large surfaces in contact with the cams 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 phosphor bronze shells with white metal linings. The pistons are aluminium and have two rings above the gudgeon pin and a scraper ring below.

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; 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, camshaft bearings, 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. A plunger type of lubrication indicator is fitted to the steering column, 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. Engraved chassis lubrication diagram is fitted on dash.


Damp-proof high-tension magneto. The magneto is mounted on a bracket with a machined face which is bolted to the near side of the crankcase and driven by a shaft from the timing case. Magneto timing is controlled by lever on the steering column. To enable the timing to be easily set, the flywheel is distinctly marked with the various settings, these markings being set to a fixed pointer.


Thornycroft-Solex type, very economical and flexible with great simplicity; a pilot jet is fitted for starting and slow running The carburettor is mounted on the off side of the cylinder block, and the mixture is warmed by the circulating water and the exhaust. Controlled by foot accelerator and by lever on steering column.

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. The radiator is built up with removable top and bottom vessels which enables new tubes to be easily fitted.

A drain cock is fitted at the lowest point so that system can be completely drained. A guard is fitted to prevent damage to the radiator. Arrangement is made to fit 3-ply diaphragm behind radiator to regulate cooling effect according to weather.

Petrol Tank

Ten-gallon (45.5 litre) brass cylindrical tank carried on the dash to WD subsidy specification, 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 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 5.27 to 1. 

Second 2.61 to 1. 

Third 1.58 to 1. 

Fourth 1 to 1. 

Reverse 8.23 to l. 

At 1,500rpm of the engine, with 36ins (914mm) pneumatic tyres and 6.25 back axle ratio the vehicle speeds are: 

On 1st speed 5mph (8kph). 

On 2nd speed 10mph (16.1kph). 

On 3rd speed 16mph (25.7kph). 

On 4th speed 25mph (40.2kph). 

Reverse 3mph (4.8kph)


Hollow propeller shaft connected with gear-box and worm shaft by fabric universal joints.

Back Axle

Cast steel casing with semi-floating differential shafts driven by overhead worm with a reduction of 6.25 to 1. Worm wheel and differential can be removed without removing the axle from the frame.


Both brakes operate on drums on the back wheels, the brakes being of the internal expanding type with drums of large diameter. Both brakes can be adjusted without the use of tools and the brake pads are exceptionally large, ensuring long life with a minimum of adjustment. The brake shoes are of the continuous band type, having no loose parts or springs to cause rattle.

Steering Gear

Of the worm and wheel type with ball thrust bearings at each end of worm shaft, which render the steering very easy. The wheel is complete so that the lever can be fixed in a new position on the spline shaft to take up backlash after the wheel has become worn. The vehicle can be turned without reversing in a road 40ft (12.2m) wide.


Channel section pressed steel. The maximum dimensions of the channel are 5ins x 3ins x 3/16ins (127mm x 76.2mm x 4.76mm). The construction is such as to give the maximum strength with minimum weight. Towing shackles are fitted, the frame being suitably cross braced to withstand shocks due to towing. 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 hardened thrusts on the pivot pins taking the weight, giving easy steering.


Of chrome steel and exceptional length, giving very easy riding. Designed to be 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.

Wheels And Tyres

Disc type bolted to cast steel hubs which run on ball bearings for the front wheels and roller bearings for the rear wheels. The wheels are suitable for: 

(a) Super-cushion singles: 100 for 720mm front, 120 for 720mm rear, fitting sizes. 

(b) NAP singles: 100 for 720mm front, 120 for 720 mm rear, fitting sizes. 

(c) Dunlop 36ins. x 6ins (914mm x 152mm) straight-sided single pneumatics on all wheels. 

All wheels are detachable and interchangeable front and rear.

Front Mudguards 

Are of ample dimensions.


11ft 6ins (3.51m).


Front wheels 60ins (152cm), rear wheels 58ins (147cm).

Ground Clearance

A large clearance, i.e. 104ins (264cm) approximately on 36ins x 6ins (914mm x 152mm) pneumatics.

General Measurements And Weights

Front axle (laden) 24cwt (1.22 tonnes). 

Back axle (laden) 49cwt (2.49 tonnes).

Chassis weight 33cwt (1.68 tonnes). 

Crankshaft diameter 2.25ins (5.72cm). 

Gudgeon pin diameter 1in (2.54cm). 

Valve diameter 1.5ins (3.81cm). 

Differential shaft diameter 2.25ins (5.72cm).


2 headlamp brackets. 

2 side lamp brackets. 

1 tail lamp bracket. 

Towing shackles.

1 14ins (35.6cm) shifter SLO 19D. 

4 double-ended spanners, 3/16ins to 0.75 ins (4.76mm x 19.1mm). 

1 tommy bar 52705. 

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 0.5ins (1.27cm) link for belt. 

1 grease-gun. 

1 instruction book. 

Tyre levers (when chassis fitted with pneumatics). 

I wheel hub extractor.

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.


Speedometer or mileometer at extra charge.


Type 2 

Introduced in late 1926, the A2 lorry was essentially an uprated A1 designed to carry a disposable load of 2 tons (2,032kg) instead of the A1's 1.5 tons (1,524kg). Thornycroft had some competition for its new 2-tonner (2,032kg) because, apart from the A2, contemporary petrol-powered 2 ton (2,032kg) lorries were manufactured by several British and foreign firms,

Specifications produced by Thornycroft for their vehicles during the 'twenties make interesting reading, not least because of the amount of detail supplied. The A2's spec was no exception, providing, for example, comprehensive chassis dimensions for the benefit of body builders, and even including metal thicknesses of chassis frame members and spring leaves! Not only this, because (for the A1 and A2 at least) we are also treated to such information as gudgeon pin diameter, valve diameter, and other internal dimensions. Specifications for both A1 and A2 chassis were practically identical, and the only discernible differences from the details provided are listed below:

  • Unlike the A1, the A2 was not offered as a subsidy vehicle;
  • Electric lighting was standard on the A2, optional on the A1;
  • 34 x 7ins (864 x 178mm) pneumatic tyres were standard on the A2; this may be compared with the A1's tyre options in the above specification;
  • The gear and final drive ratios were the same for both the A1 and A2. However these two vehicles reached different speeds in each gear at a given engine rpm, thanks to tyre size differences;
  • The A2's increased carrying capacity gave different axle loadings for both the A1 and A2. However, weight distributions were little different, namely 28.3 per cent (front) and 71.7per cent (rear) for the A2, and 29.5per cent and 70.5per cent for the A1;
  • The A2's chassis weight was 1.7 tons (1,727kg) compared to the A1's 1.65 tons (1,677kg);
  • There was slight difference in track, depending on tyres used;
  • The A2 and A1 had different petrol tank designs, although both tanks had a 12 gallon (54.6 litre) capacity. 

In addition to the differences between the A1 and A2 listed above, the A2's rear axle was strengthened in comparison to the A1's, to cope with the extra weight. Also, the A2 had modified brakes. The two items just mentioned would have contributed to the A2's different chassis weight (1.7 tons (1,727kg)). Also, assuming the chassis weights in the Thornycroft specs are given for vehicles in standard trim, then a further contribution to the weight difference would have come from the A2's lighting set.



Chassis diagram 91kb pdf