2¾hp. Machines for the 1920 Isle of Man T.T.

An article from 'Motor Cycling', 19th May, 1920:

2¾hp. Racers Develop 10hp.
Full details of high efficiency junior T.T. machines

The Stevens brothers, than whom few people in the motorcycle trade have more experience in engine building, have produced an extremely efficient 2¾hp. racer for this year's Tourist Trophy. One machine will be ridden in the senior event, and six in the junior, the riders being Eric Williams (who will also ride a Sunbeam in the senior), Howard Davies, Bert Haddock, Harris, Harold Heath, and O. Wade.

The A.J.S. racer is a four-speed machine, two sprockets of different size being mounted on the engine shaft, and driving thence to a double sprocket machined out of the solid on the countershaft, where an ordinary two-speed gear is fitted. The latter is controlled by a lever mounted on the offside of the tank, whilst the engine-shaft gear, which is changed by clutching up either sprocket, by means of internally cut dogs, with the engine shaft, is operated through a rod and Bowden wire arrangement down the right handlebar.


     The T.T. A.J.S. simply bristles with innovations. The new type of gear is
     shown in this view, as is the unique method of saddle springing.		 At present the gear ratios obtainable are 4⅝, 5½, 7¼, and 8½ to 1, though these may be modified later after practice in the Isle of Man.

The top gear is used down long slopes with a view to saving the engine, which would otherwise be turning over at a terrific speed.

 

An astounding r.p.m. has been obtained, and 10hp. developed by the dynamometer test by the use of very light working parts, a very high compression, and large overhead valves set at an angle of 45 degrees to each other.

A single ring drilled steel piston is used in preference to an aluminium one, as the latter type broke up when tested at Brooklands. The piston is machined out of a solid bar of steel, as are the flywheels. The connecting rod is a wonderful piece of work, being machined all over, and as light as it is possible to make it without risk of fracture.

A detachable combustion head with large and smoothly-polished surface and ports, carries the inlet and exhaust valves, the exhaust port being a good deal larger than the inlet.

The cylinder head is held down by a metal strap, which is anchored in the crankcase and provides a very neat, light and strong fixing.

The overhead rocker arms and their supports are all made of the highest grade steel, the arms being operated by hollow tappet rods.

The compression used is 5½ atmospheres. Some trouble has been experienced in discovering a sparking plug, which will stand up to the work, but the Cooke plug has now been adopted after prolonged tests.

The cylinder, like the head, is made of cast iron. It is machined inside and out, the radiating fins being a particularly fine piece of work.
The tank holds two gallons of petrol and is provided with extra large filler caps. The oil filler communicates with its tank, which is clipped to the rear down tube and holds half a gallon of oil, via a large passageway, and is arranged thus to facilitate filling, the oil being pumped into the tank by the rider himself at the depot.

 

The two pictures show the novel construction of the engine. The placing of the valves at an angle in the head enables those of large diameter to be used. The top picture also shows the two-speed countershaft gear which is employed conjointly with an Enfield type of gear.

An article from the 'Motor Cycle', 10th June, 1920:

A.J.S. Machines for the T.T.
Four-speed gears applied to sturdy machines having high efficiency single-cylinder engines

Considerable interest will be taken in A.J.S. machines, which besides defending their title in the junior race next Tuesday are the only 350c.c. engines entered for the senior event.

Frame, gearbox, forks, and brakes are A.J.S. standard practice, but the engine and certain features of the transmission are new. The single-cylinder engine has a bore and stroke of 74 x 81 mm respectively. To all intents and purposes the crank case is standard, but steel flywheels are used and the cast iron cylinder has a detachable hemispherical head in which overhead valves are mounted inclined from the centre line at approximately 45 degrees.


Driving side of the A.J.S. which will defend the junior trophy.

 These valves are operated through standard timing gear and long push rods, overhead rockers being carried in brackets bolted to the head.

The fixing of the cylinder head is unusual and very ingenious, as it is held down by a flexible steel strap passing over the head of the engine and tightened by turnbuckles on either side of the crank case.
Unorthodox Transmission

A very light steel piston is employed having two rings at the top and a series of oil grooves, the lower part of the skirt is copiously drilled.

The transmission from the engine shaft to the gearbox is somewhat unusual, two equal sized sprockets are machined on the clutch ring, and mounted loosely on an extension of the crankshaft are two sprockets which differ slightly in size.

Either of these sprockets may be dogged to the shaft at will, a control being operated from the handle bar.

When the smaller of these sprockets is being utilised for the drive, the normal high and low gear are obtained by means of the countershaft gearbox, but the larger sprocket can be used for long descents, and gives slightly higher ratios so as to prevent over-running of the engine.
  The cylinder head of the A.J.S. is held down by a steel
  strap, and oil is supplied by a handle-bar controlled
  pump from a tank behind the saddle tube.

Driving side of the T.T. A.J.S., showing the housing for the engine shaft gear and the inclined overhead valves.		 The saddle tank contains two gallons of petrol, and the entire oil supply is carried in a tank behind the saddle tube. A somewhat unusual method of filling this tank is employed, since a large diameter pipe is carried upwards through the petrol tank so as to bring the filler cap into a normal and easily accessible position.

Lubrication is effected by a spring pump controlled from the handle bar, the flow being regulated by a sight feed drip. It is just possible however, that this drip will not be used during the race.

From the illustrations, it will be noticed that the cylinder is stayed to the front down tube. The machine is fitted with a Brooks saddle, Amac carburettor, Hans Renold chains, Dunlop tyres, and Thomson-Bennett large size magneto.

It is a most workmanlike mount, which has been designed rather with the idea of a high efficiency machine which will stand up to the work, than with the idea of saving weight.

A Sporting Chance

There is no doubt that the A.J.S. stands an excellent chance of retaining its title since there are few machines which have been so thoughtfully designed and well constructed. It is unfortunate, therefore, that T. H. Haddock and N. C. Heath, two of the T.T. riders of this model, should have suffered accidents during preliminary practice. Whether these accidents will prevent their riding in the race is not certain, but we understand the A.J.S. firm have other good riders in reserve.
That one of these machines has been entered in the senior event gives the A.J.S. a sporting chance of winning both events; in any case, its performance in competition with machines of 30% larger capacity will be watched with great interest.
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