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Thornycroft Type JJ Lorry 0.A

In addition to its rigid six-wheelers, Thornycroft entered 1927 producing seven types of four-wheel lorry, listed below. These vehicles covered the load range 1.5 tons to 6 tons (1,524kg to 6.096kg) and were all powered by large in-line L-head four-cylinder engines:

Lorries powered by the 3,620cc FB/4 engine (36bhp @ 1,500rpm):

  • Type A1, 1.5 tons (1,524kg);
  • Type A2, 2 tons (2,032kg), the A1’s bigger brother.

Lorries powered by the 5,420cc HB/4 (46bhp@1,500rpm):

  • Type KB, 3 tons (3,048kg);
  • Type PB, 4 tons (4,064kg).

Lorries powered by the 6,970cc BB/4 (58bhp@1,600rpm):

  • Type J, 4 tons (4,064kg);
  • Type Q standard wheelbase, 5 tons (5,080kg);
  • Type Q long wheelbase, 6 tons (6.096kg).


Thornycroft entered 1927 producing the 1.5 ton A1 and 2 ton A2. Shown here is a stylish A series tanker.

Thornycroft probably decided that keeping two 4 ton four-wheelers in production, the Types J and PB, was uneconomic duplication. Whilst the firm was still taking orders for the J in 1927, the type had been in production since 1912, albeit with improvements added over the years. The upshot was that Thornycroft dropped the venerable J and retained the more recent PB to meet demand for a 4 tonner (4,064kg). In addition, Thornycroft introduced the Type CC to exploit an emerging market for a 7 ton (7,112kg) four-wheeler.

The JJ was introduced in 1927 as a larger version of the J, incorporating a number of improvements, but retaining the J’s two-wheel braking system. The JJ duplicated the 5 ton (5,080kg) capacity of the standard wheelbase Type Q, so further rationalisation was needed and Thornycroft’s four-wheel model range for 1928, shown below, was the logical result:

Lorries powered by the 3,620cc FB/4 engine (36bhp @ 1,500rpm):

  • Type A1, 1.5 tons (1,524kg);
  • Type A2, 2 tons (2,032kg).

Lorries powered by the 5,420cc HB/4 (46bhp@1,500rpm):

  • Type KB, 3 tons (3,048kg);
  • Type PB, 4 tons (4,064kg).

Lorries powered by the new 6,970cc MB/4 (60bhp@1,800rpm):

  • Type JJ standard wheelbase, 5 tons (5,080kg);
  • Type JJ long wheelbase, 6 tons (6.096kg);
  • Type CC, 7 tons (7,112kg).

In the meantime, Thornycroft announced in 1927 that all its engines should have in-unit gearboxes and clutches. The FB/4 and HB/4 already met this requirement, however the BB/4 which powered the J and Q had a separate gearbox and being therefore obsolete was replaced by the unit MB/4 engine. The latter, introduced in 1927, had the same bore and stroke as its BB/4 predecessor, and a small increase in power. In time, unit engines became almost universal, allowing good alignment between engine and gearbox. In addition, the unit MB4’s clutch was fully-enclosed in the engine’s rear bell-housing extension and was therefore protected from dust, dirt and moisture, prolonging clutch life and hence reducing maintenance costs. By contrast, the clutch of the J’s superseded BB4 non-unit engine was housed within the engine’s external flywheel and was thus semi-exposed and more vulnerable to the elements. A further advantage of the JJ’s unit MB/4 engine over its BB/4 predecessor was a reduction in weight due to unit construction, and the construction also allowed more rapid engine removal and replacement for overhaul. These benefits reduced operating expenses, pushed down the cost of goods and thereby contributed, with other technological developments nationwide, to economic growth.

The BB/4 engine failed to meet Thornycroft’s policy of producing engines with in-unit clutches and gearboxes

Thornycroft made some demonstration runs of the JJ to important commercial vehicle operators in 1928, over the normal routes of each operator with the vehicles under the observation of the user’s own representatives. The vehicles were used on journeys to and from various destinations up to 150 miles (241km) from London, e.g. Brighton, Maidstone, Cambridge, Canterbury etc. The vehicles were carrying full loads of 6 tons to 6.5 tons (6,096kg to 6,604kg) with trailer loads averaging over 4 tons (4,064kg). As these vehicles had to work hard to shift these loads, then, under the circumstances, average speed and petrol consumption of around 18mph (29kph) and 6mpg (47.1 litres per 100km), respectively were creditable. It should also be recognised that the low compression ratios of the day, 4.4 to 1 for the JJ’s MB/4, militated against petrol economy. As fuel quality and engine design improved, CRs were raised over time.

The JJ was improved during its production life. Early models were offered with solid or pneumatic tyres, but later models had pneumatics only, in keeping with the general demise of solid tyres. Iintroduced in 1928, the lwb JJ could tow a trailer carrying a 6 ton load (6,096kg) i.e. 12 tons total load (12,192kg), although this combination must have been woefully underpowered even by the modest standards of the late ’twenties. By 1929, there were no less than six JJ options including the normal wheelbase, lwb and forward control each of which was built in 5 ton (5,080kg) and 6 ton (6,096kg) form. The JJ went out of production in 1930.

Introduced to replace the BB/4 engine in 1928, the MB/4’s clutch and gearbox were integral with the engine. Photo: Nick Corrie


Specification of Thornycroft 5 and 6 ton Chassis (5,080 and 6,096kg) Type "JJ" (Petrol) December 1930

36.2hp (RAC) type "MB4", watercooled four-cylinder petrol engine, bore 4.75ins (120.7mm) x stroke 6ins (152.4mm), developing 60bhp at 1,800rpm. Compression ratio is 4.4 to 1. The four cylinders are of the monobloc type with two detachable heads. The valves have double springs, and the Thornycroft standard roller tappets, with accessible adjusting nuts, are employed. The pistons are of cast iron with three piston rings, but, if desired by the purchaser, aluminium pistons can be supplied. The connecting rods are of duralumin, the caps being fastened by two bolts. Big end bearings are gunmetal shells with white metal linings. The crankshaft is carried in three main bearings.

Lubrication of engine
Four gallons (18.2 litres) of oil are carried in the sump for lubrication and the oil pump is submerged in the oil. It supplies the oil to a channel which runs end to end of the engine at a pressure of 15lb/in2 (1.034 bar). From this oil channel the lubricant passes to the main bearings and through a drilled crankshaft to the big-ends, the cylinder walls and small-end bushes being lubricated by splash. The camshaft bearings are pressure-fed and the surplus oil passes to the timing gear. No copper pipes are incorporated in the lubrication system, except the external pipe to the pressure gauge on the dashboard. A dipstick is fitted with maximum and minimum marks and a test cock is fitted in the crankcase to indicate the high level point of oil. An oil filter is fitted.

The magneto is mounted on a platform on the crankcase, which is driven from the front end of the crankshaft through gear wheels. Provision is made for interposing a dynamo between the driving gear and the magneto when an electric lighting system and engine starter are fitted. Advancing and retardation of the ignition are effected automatically.


A Zenith carburetor with a hot-spot induction manifold ensuring complete vaporization. The throttle is controlled by an accelerator pedal, a setting for slow running being obtainable through a lever on the steering column.

Cooling system
A propellor type pump is run in tandem with the cooling fan. Tension of the fan belt is regulated by means of an adjustable flange on the driving pulley.


This is of the vertical gilled tube type. The radiator has removable top and bottom vessels, enabling new tubes to be easily fitted.

In unit with the engine.

Mounted on the rear end of the engine crankcase, the gearbox provides four forward speeds and a reverse.

Transmission. The power is taken from the gearbox to the rear axle by propeller shaft.

Driving axle unit
Drive is transmitted to the differential unit by an overhead worm. Final drive ratio is 8.25 to 1. Access to the worm gear is obtained by withdrawing the fully-floating drive shafts.


Drum brakes are fitted to rear wheels. Front wheels are unbraked. A transmission brake is also fitted.

Wheels and tyres
Detachable disc type wheels bolted to steel hubs running on taper roller bearings.

Pneumatic tyres, 40ins x 8ins (101.6cm x 20.3cm) front and rear.


Maximum track. 6ft 1.5ins (1.87m)

  • Standard wheelbase vehicle:
  • Wheelbase. 14ft 6ins (4.42m).
  • Frame height. 2ft 9ins (0.84m).
  • Body space (dash to end of frame) 18ft 10.5ins (5.75m).
  • Long wheelbase vehicle:
  • Wheelbase. 15ft 6ins (4.72m).
  • Frame height. 2ft 9ins (0.84m).
  • Body space (dash to end of frame) 19ft 10.5ins (6.06m).
  • Forward control vehicle:
  • Wheelbase. 13ft 0ins (3.96m).
  • Frame height. 2ft 9ins (0.84m).
  • Body space (dash to end of frame) 15ft 2ins (4.62m).
  • Chassis weight.
  • Standard wheelbase and forward control. 73.25cwt (3,721kg)
  • Long wheelbase. 73.5cwt (3,734kg)