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1903 Thornycroft 10hp car: Specification



The car is powered by an adapted A-type Thornycroft marine engine, known as the AZ in its automotive application.


The twin-cylinder AZ engine is a four-stroke in-line vertical unit, front-mounted in the fore and aft direction.


Bore and stroke are 4 x 4.375ins (101.6 x 111.1mm), giving a swept volume of 1,788cc and an RAC rating of 12.8hp. The iron cylinder block, cylinder heads and water jacket are cast as a single item, which is bolted to the crankcase.


Cast-iron pistons are fitted, and each is fitted with four cast-iron piston rings. As was normal in 1903, oil-control rings are not fitted.

Connecting rods

Cast steel.


The two bearing, two-throw steel crankshaft has cranks set at 180 degrees to each other.


The aluminium crankcase has a removable bottom cover; this can be removed without disturbing the main bearings.

Valve gear 

Each cylinder is fitted with two poppet valves on the left side of the engine, arranged as inlet-over-exhaust. The automatic overhead inlet valve is opened by atmospheric pressure during the induction stroke, while the side exhaust valve is operated by a camshaft located in the crankcase. Cam lobes are detachable from the camshaft, following marine practice. The camshaft is gear-driven from the front of the crankshaft.


Spark ignition is fitted, with one spark plug per cylinder. Each of two high-tension (HT) trembler coils mounted on the dash supplies one spark plug. A 6-volt battery mounted under the front seat supplies the low tension (LT) side of each trembler coil through the original French Nil Melior LT distributor. The battery is charged by a dynamo, which is belt-driven by a pulley mounted on the back of the crankshaft. The ignition system gives a very strong spark, capable of jumping a test gap of several inches. Battery charging is controlled by a regulator mounted under the front seat. A centrally located quadrant on the steering wheel houses the manually-operated ignition advance-retard lever.

Fuel system 

The AZ is a naturally-aspirated engine, and receives petrol-air mixture from a Thornycroft patent carburettor mounted on the right, low down on the engine. The carburettor is connected to the induction manifold through a long induction pipe, and is gravity-fed with petrol from a tank under the front seat. A centrally located quadrant on the steering wheel houses the throttle lever. In the absence of a choke, a few drops of petrol are administered through a tap into a priming cup in the inlet valve chest.

Standard pump petrol.


The engine is lubricated on the total loss principle as the pistons are not fitted with oil control rings. The sump is filled with oil to a pre-determined level to provide splash lubrication, and the level is maintained on the move by a dash-mounted manually-operated oil dispenser with an inbuilt reservoir. The dispenser is operated by pulling up a plunger and then releasing it, after which it descends slowly while a predetermined quantity of oil is supplied to the sump. After a few miles the plunger is lifted again and the process is repeated. After a journey, surplus oil in the sump, if any, is drained off from the drain tap.

Exhaust system

An exhaust pipe from the exhaust manifold runs rearwards under the car to a cylindrical silencer, in the conventional manner. Unusually, the tailpipe is connected to the underside of the silencer, and turns through 90 degrees to discharge horizontally behind the car.

Cooling system

The engine is water-cooled. A water pump, gear-driven from the front of the crankshaft, circulates water through the engine water jackets and a front-mounted radiator. The radiator comprises gilled piping folded into a zigzag several times in order to fit into the aperture at the front of the car. A header tank is mounted on the scuttle just behind the engine. There is no cooling fan. 


No self-starter is fitted, and the engine is started manually by a front mounted starting handle engaging with a forward extension of the crankshaft. The ignition is sufficiently retarded before hand cranking in order to avoid kickback.


The engine's emissions are cleaner than those of modern engines, possibly due to its low rpm giving time for complete combustion.



The clutch is of the expanding friction type with a cylindrical pressure surface incorporated with the flywheel, the latter being mounted externally at the back of the engine. Fibre friction pads are spring-loaded to press outwards against the pressure surface when drive is transmitted. The clutch is operated by the left pedal.


The gearbox is mounted behind the clutch, and has three forward speeds and a reverse. The gear lever is mounted in a quadrant outside the car and is operated by the driver's right hand. The gearbox is filled with oil; in addition, a dash-mounted device provides hand-applied forced-grease lubrication to the gearbox bearings. Greasing can be carried out on the move.

Final drive 

Gearbox output is transmitted to an oil-filled differential mounted in the rear axle, through a propeller shaft supported on universal joints.

Chassis - the chassis is of pressed steel.

Other data


Wooden artillery-type wheels are fitted, with beaded edge pneumatic tyres. There is no spare wheel.

Axles and suspension

Beam front and rear axles are suspended from semi-elliptic leaf springs. Front springs are anchored to the chassis. The front ends of the rear springs are anchored to the chassis, while their rear ends are anchored to an inverted transverse leaf spring, itself attached by its centre to the chassis.


The car has right-hand drive with high-geared steering through a worm and wheel steering box. There is a small diameter steering wheel and the raked steering-column is unsupported.


As was normal in 1903, the car has no front brakes. Mechanically-operated contracting brakes are fitted to the rear wheels and to the front of the differential (transmission brake). Wheel brakes comprise friction material contracting against V-shaped drums, and are applied by an external brake lever operated by the driver's right hand. The right pedal applies the transmission brake, which is of the metal-to-metal type.


The car has an open, four-seater, tonneau body. The body is of wood construction, comprising laminated hardwood panelling attached to an ash frame. There are no side doors. Front occupants enter the car through side cut-outs in the bodywork, while rear passengers enter through a rearwards-facing door. The interior of the car is upholstered. There are no windscreen, side or rear windows. The metal, louvred, bonnet opens rearwards on scuttle-mounted hinges. Paraffin-fired headlights are fitted. A tail light is also fitted.


There is a bulb-operated horn on the driver's side of the car.

Weight - 1,456lb (660.4kg).


Wheel base 78ins (1.98m), front and rear track 48ins (1.22m), tyres 815 x 105mm

Rear axle ratio 3.9 to 1.

Performance (estimated)

Speed at 1000 rpm

Speed at 1000rpm: 24.5mph (39.4kph) 

Petrol consumption: 20mpg (14.1 litres per 100km).

Price - the car cost £460 new in 1903.