The Mastiff RSW was introduced in 1932 as a normal control 7 tonner (7,112kg). Designated as the SD model, the vehicle was offered in two versions, the SD/MB4 with a 60bhp MB4 four-cylinder petrol engine and the six-cylinder SD/WB6 powered by a 78bhp WB6 engine. Both MB4 and WB6 were side-valve engines. As usual with Thornycroft RSWs, the Mastiff had one front axle and two rear axles, the latter being used in conjunction with Thornycroft’s patented rear suspension system.
Note the starting handle and forward-facing exhaust exit on this 1933 Mastiff tanker.
Gearbox and engine formed one unit, and power was taken from the gearbox to the overhead worm of the first of both two rear axles. The second rear axle was undriven (i.e. a trailing axle), marking the Mastiff as not being intended for off-road work. Also, the Mastiff did not have the two-speed auxiliary gearbox with which some of Thornycroft’s other vehicles were fitted, including the similar Amazon RSW – the latter’s rear axles, also, were both driven.For 1933, the Mastiff was uprated to 7.5 tons (7,620kg) and a diesel version was added, powered by the four-cylinder CIND4 diesel giving 82bhp@1,800rpm. Both existing petrol versions were retained, but the six-cylinder model was given the new 99bhp AC6 overhead-valve engine replacing the 78bhp side-valve WB6 introduced to the Thornycroft range in 1929.
The Mastiff failed to survive a pruning of Thornycroft’s RSW range, and, although introduced in 1932, was no longer available by 1934.
Mastiff “SD/AC6” rigid six-wheeled freight chassis
To carry a net load of 7.5 tons (7,620kg) with a body allowance not exceeding 20cwt (1,016kg).GeneraL. Combined with the latest developments in design and construction, ease of maintenance and accessibility have been given particular attention throughout.
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 almost entirely eliminated.
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.
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.
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.
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.
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.
By high tension, automatic-advance magneto. The magneto is located at the front of the engine.
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.
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.
A vertical gilled-tube radiator of distinctive appearance, with detachable top and bottom water vessels, mounted on rubber buffers.
A single-plate dry clutch of large diameter and area, with specially-prepared fabric surfaces. The lightness of the driven member and the efficiency of the clutch stop make gear-changing simple and rapid.
Of unit construction, providing four forward speeds and reverse, top gear being direct drive. The gear shafts, which are 2½ins (63.5mm) diameter, run in large-size roller bearings, and the gear wheels, with teeth 1.3125ins (33.34mm) wide, are of special contour and ground to obtain silent running. A speedometer drive is incorporated with the gearbox.
The reduction ratios are as follows:
Change Speed Control
Change-speed lever mounted direct on gearbox, light in operation, permitting easy engagement of gears.
The drive is transmitted from the gearbox by a hollow propeller shaft with enclosed metallic universal couplings. An intermediate propeller tube is interposed between the gearbox and the propeller shaft to the driving axle, the rear end being supported by a ball bearing flexibly attached to one of the frame crossmembers.
A nickel steel casing with full-floating differential shaft of 2ins (50.8mm) diameter, driven by overhead worm with a reduction ratio of 7¼ to 1. For exceptionally difficult conditions of service an alternative reduction ratio of 8¼ to 1 can be provided, without extra charge, if specified when the chassis is ordered.
A second (trailing i.e. undriven) axle forms the rear support for the springs of the bogie.
A 3ins x 3¼ins (76.2mm x 82.3mm) section axle body of medium carbon steel forged in one piece. The 2.1875ins (55.56mm) diameter swivel arms are of nickel steel with taper-roller thrust bearings on the pivot pins, taking the weight and giving easy steering. The track rod is adjustable.
Internal expanding brakes are fitted to all six wheels. The foot brake, assisted by a powerful vacuum-servo cylinder, operates on 20ins (50.8cm) drums on all four wheels of the bogie and on the front wheels, while the hand brake acts on the four bogie wheels only.
All the brake gear is easily adjustable, but the facings consist of large fabric pads ensuring long life with a minimum of adjustment.
Steering Gear. Worm and wheel type with taper-roller bearings. The worm wheel is complete, thus permitting a fresh sector of the wheel to be brought into service should backlash develop through wear.
The vehicle has a turning circle of 70ft (21.3m) when fully laden. This will change when alternative tyres are fitted.
Lubrication of Chassis
By grease-gun and oil-gun, the nipples being grouped in accessible positions.
A pressed steel frame of deep channel section, reinforced in the centre. Channel-type crossmembers with wide gussets correctly placed give an extremely rigid frame construction without employing excessive weight of material.
A 40-gallon (181.8 litres) steel tank with petrol gauge is fitted on the side of the chassis frame, where it is easily accessible for filling. A vacuum tank draws the petrol from the main tank and delivers it by gravity to the carburettor.
Four inverted semi-elliptic springs independently pivoted at their centres to brackets rigidly attached to the chassis frame, designed under Thornycroft Patent No. 259,322, to give even distribution of weight on all four wheels of the bogie under all conditions, irrespective of the relative movements of the axles.
The springs form the only connection between the axles and the frame, no torque tubes or radius rods being employed, the outstanding feature of this design being that the torque reaction is transmitted to the frame through the road springs, which ensure a cushioning effect, and thus relieve the frame of shocks.
The suspension is designed to permit a 9ins (22.9cm) difference in driving axle levels and also to allow either axle to tilt to an angle of 13 degrees (when one wheel will be approximately 18ins (45.7cm) above the other) without causing any distortion of the springs. Rubber buffers fixed above and below the axles prevent excessive movement.
Made of silico-manganese steel, ensuring easy riding. The front springs, which are designed to be approximately flat under normal load, are secured to the axle by our patented type of relieving plate (Patent No. 199,522), which enables the holding-down bolts effectively to withstand the stresses due to flexing of the springs without suffering undue elongation.
In order to relieve the central bolt of shear stress which would otherwise be caused by horizontal forces due to road shocks, etc., the holding-down bolts are inclined to one another, and special ears are provided on the eye-plate to Grip Them.
Wheels and Tyres
38ins x 9ins (96.5cm x 22.9cm) straight-sided pneumatic tyres are fitted to disc wheels, single on all axles. Alternatively, 11.25ins-20ins (28.6cm-50.8cm) low pressure tyres, single on all axles, may be fitted at extra charge.
Exceeding 5 tons (5,080kg) weight unladen. £48.
Standard Chassis Weight with 38ins x 9ins (96.5cm x 22.9cm) tyres, battery, standard lamps and dynamo, minimum oil in engine, gearbox and driving axles, but excluding petrol, water, tools, spare wheel and carrier: 4 tons 13cwt (10,416lbs/4,725kg)
Body allowance:1 ton (2,240lbs/1,106kg)
Weight of chassis plus body allowance 5 tons 13cwt (12,656lbs/5,741kg)
Standard Chassis Equipment
A 12 volt electric lighting set with dip and switch headlamps, two wing lamps, combined tail and “stop” light, 105 amp-hour battery, electric horn operated from centre of steering wheel, electric starter, engine speed governor, speedometer and mileage recorder, spare wheel with tyre and carrier, mechanical tyre pump, and a comprehensive kit of tools.
The following may be supplied at extra charge:
Information for these pages comes from contemporary Thornycroft technical and other data, as well as contemporary copies of The Commercial Motor.
Mastiff outline diagram 107kb pdf