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Stag

Thornycroft’s RSW range in 1933 comprised six models, covering load capacities from 3 tons to 12 tons (3,048kg to 12,192kg). The heaviest model was the Jumbo (11 tons to 12 tons/11,176kg to 12,192kg), without off-road capability. This vehicle had a very brief innings, being listed only for 1933. Also listed for 1933 was the heavy Dreadnought (10 tons to 11 tons/10,160kg to 11,176kg), with limited off-road capability.

This majestic and pristine Stag RSW is quickly recognised by its sloping front and radiator

In 1934, Thornycroft replaced both the Jumbo and Dreadnought with the Stag, without off-road capability and with a carrying capacity of 10 tons to 12.5 tons (10,160kg to 12,700kg). In addition to the Jumbo and Dreadnought, Thornycroft discontinued two other RSWs, and of the six listed for 1933, only two survived into 1934 alongside the new Stag.

Thornycroft RSWs listed for 1934 were as follows

  • Tartar (3.25 tons/3,302kg, increased from 1933), with two driven rear axles, a two speed auxiliary gearbox and off-road capability
  • Amazon (6 tons/6,096kg), with two driven rear axles, a two speed auxiliary gearbox and off-road capability
  • Stag (10 tons to 12.5 tons/10,160kg to 12,700kg), with one driven rear axle, a two speed auxiliary gearbox and no off-road capability only. This was a new vehicle in 1934

Intended for main-road heavy haulage, the Stag was introduced at the Commercial Motor Show at Olympia, London, in late 1933 for the 1934 model year. Its appearance was imposing and attractive, due to its sheer size and its characteristic sloping radiator as well as a more modern-looking cab than its predecessors. It came with forward control and was offered with diesel and petrol options, both with six-cylinders. The Stag’s 7,749cc AC6 petrol engine developed 104bhp, slightly more than the engine’s 99bhp rating of 1933 in other vehicles. The Stag’s diesel option was powered by the enormous CIND6 of 11,339cc giving 123bhp@1,800rpm. The two-speed auxiliary gearbox was fitted as standard, and customers could specify either a Thornycroft auxiliary box or a German-built Maybach unit.

Unfortunately for the Stag, Thornycroft was suffering from financial problems when the vehicle was introduced and for a while thereafter. Also, UK demand for RSWs was falling, from 1,721 vehicles in 1934 to 748 in 1936. It was hardly surprising, therefore, that Thornycroft pruned its RSW range to only three types for 1934. However, there was more pruning to come, resulting in the Stag being the only RSW listed in the UK by Thornycroft for 1935 and 1936.

For 1936, Thornycroft replaced the Stag’s enormous CIND6 diesel engine with the much more compact six-cylinder DC6, giving 99bhp from 7,883cc. However, this was the Stag’s swansong, and production was discontinued after 1936. With the demise of the Stag, no Thornycroft RSWs were offered on the home market for 1937 and 1938, although they were built for export.

 

Stag “XE/AC6” rigid six-wheeledfreight chassis

To carry a net load of 10-12 tons (10,160-12,192kg)

March 1934

Engine

Six-cylinder 99bhp at 2,100rpm, type AC6, watercooled petrol engine, 4.375ins (111mm) bore, 5.25ins (133.4mm) stroke. Cubic capacity 7,749cc. Three point suspension of special construction, stresses in crankcase due to frame distortion are thus entirely eliminated.

Cylinders
Monobloc construction assisting rigidity of engine and fitted with special wet cast iron liners. The cylinder head is easily detachable for decarbonising and grinding-in the valves.

Valve Gear

Overhead valves operated by push rods and rockers from single camshaft housed in the crankcase, and driven from the front of the engine by a triplex chain with an adjusting device, rendering the drive extremely reliable. The tappets are easily removed through three large covers on the side of the cylinder block and access to the rockers is obtained by removal of an aluminium cover secured by hand screws.

Crankcase

A high-grade casting of special alloy, designed to give extreme rigidity with minimum weight. The cylinder block, crankcase and main bearing caps, reinforced with steel keeper plates, are secured together by through-bolts and stiffening pads at the points of maximum stress. The bottom half is easily detachable for the inspection and taking-up of the main and big end bearings.

Crankshaft

A dynamically and statically balanced nickel-chrome steel crankshaft of robust design, machined all over and carried in seven white-metal lined bearings of 2.875ins (73.03mm) diameter. A very efficient vibration damper is fitted to maintain smooth running over the whole speed range.

Lubrication

Force-feed lubrication, special attention being paid to the filtering of the oil. From an efficient external oil filter, which can be removed without loss of oil or the necessity to drain the sump, the oil passes to the main and connecting rod bearings, also to the camshaft bearings and camshaft thrust bearing. Oil is fed to the overhead valve gear, under reduced pressure, through the hollow rocker shaft. Provision is made for cooling the oil by ribs cast on the filter and on the sump, the former being placed in the air stream from the fan. The sump, which has a capacity of 3½ gallons (15.9 litres), is easily drained by unscrewing a single nut on the side of the engine and without the necessity of crawling underneath the chassis.

Ignition

By high tension, automatic-advance magneto. The magneto is located at the front of the engine.

Carburettor

Float-feed automatic type with special starting jet in addition to a pilot jet for slow running. Induction manifold heated by “hot spot” from exhaust. The throttle is controlled by accelerator pedal and an adjustable lever on the steering column. A specially designed governor is driven from the camshaft.

Cooling System

By pump, direct driven from the engine, and fan driven from a pulley on the dynamo shaft. The pump is of the impeller type and when out of action will not impede thermo-syphon cooling.

Radiator

A vertical gilled-tube radiator of distinctive appearance, with detachable top and bottom water vessels, mounted on rubber buffers.

Clutch

A single-plate dry clutch, with stop.

Gearbox

Of unit construction, providing four forward speeds and reverse.

Gear

Ratio inc: final drive

Fourth

8.75

Third

13.3

Second

21.0

First

33.3

Reverse

48.0

Above ratios are with auxiliary gearbox in direct drive.

Auxiliary Gearbox
Two speed unit operated by a separate lever, providing direct drive or further reduction of 2.5 to 1, which affords an additional range of exceptionally low gear ratios for extremely bad conditions.

Transmission
The drive is transmitted from the main gearbox through a tubular shaft with one enclosed metallic universal joint to the auxiliary gearbox, thence by a tubular propeller shafts with two similar joints, to the leading rear axle.

Driving Axle
The first of the two rear axles is the driving axle, incorporating two fully-floating half shafts of 2¼ins (57.2mm) diameter, an overhead worm gear driven by the propeller shaft, and a differential.

Front Axle

One piece type.

Brakes

The foot brake operates through a Dewandre three-cylinder vacuum-servo system an all six wheels. The hand brake operates on all four bogie wheels. The diameter of all drums is 17ins (43.2cm). Front shoes are 3ins by 14½ins (7.62cm by 36.8cm), rear shoes are 4¾ins by 12⅝ins (12.1cm by 32.1cm). Frictional area of foot brake is 651sq ins (4,200sq cm).

Steering
Left and right turning circles are 72 ft and 73 ft, respectively (21.9m and 22.3m). There are four turns of the steering wheel from lock to lock.

Frame
A pressed steel frame of maximum section 9ins x 4ins x 0.3125ins (22.9cm x 10.2cm x 0.794cm).

Petrol Tank

Two 50 gallon (227.3 litres) tanks are carried on either side of the chassis frame.

Rear Suspension
Four inverted semi-elliptic springs independently pivoted at their centres to brackets rigidly attached to the chassis frame

Wheels and Tyres
Pressed steel wheels shod with 42ins x 9ins (106.7cm x 22.9cm) tyres at front, and 13.5ins-20ins (34.3cm-50.8cm) at rear, single on all axles.

Weights.

  • Chassis 5 ton 7¾cwt (12,068lbs/5,474kg)
  • Body allowance1 ton 6cwt (2,912lbs/1,321kg)
  • Pay-load inc:2 passengers 12 ton 0cwt (26,880lbs/12,193kg)
  • Driver 0 ton 1¼cwt (140lbs/63.5kg)
  • Fuel, spare wheel,etc 0 ton 3½cwt (392lbs/178kg)

 

Information for these pages comes from contemporary Thornycroft technical and other data, as well as contemporary copies of The Commercial Motor.

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Stag outline diagram 47kb pdf