Overhead Valve production machine

An article from 'The Motor Cycle', 2nd November, 1922:

Overhead Valve A.J.S. Available
New 349c.c. model based on famous T.T. winning type

More than once criticisms have been heard owing to the fact that the makers of the A.J.S. have repeatedly won the Tourist Trophy race with an overhead valve model which was not available on the open market.

Nevertheless the manufacturers acted wisely, for they thought that until the experimental machines had been reduced to a form in which they were safe in the hands of the public, it was better not to sell them broadcast. Now, however, the latest developments render the o.h.v. model as available as the popular side valve type.

A new A.J.S. model which is practically a replica of the successful T.T. machine.

At the forthcoming Olympia Show there will be staged a super sports model with an engine based on the T.T. A.J.S. machine. The overhead valve mechanism will be almost exactly similar, except for the fact that somewhat longer valve guides are employed so as to reduce wear at this vital point. These valves give a clear port opening equivalent to one and nine sixteenths inches, and a lift of five sixteenths of an inch. They are operated through hollow push rods with an independent return spring for the rocker gear. Recent modifications include an increased heads cooling area with vertical fins arranged to lie in the same direction as the main air current. The detachable head is held down by a round section steel strap, semi-circular in form. As might be expected, the head and cylinder joint are formed on the patented A.J.S. lines.

Four-ringed aluminium piston

Another innovation since the Tourist Trophy races takes the form of an aluminium piston, though the four very narrow rings above the gudgeon pin are retained. A hollow gudgeon pin is fixed in the piston bosses by means of a split pin, a bronze bush being fitted to the small end of the connecting rod; the connecting rod itself is a nickel chrome forging, heat treated and machined all over. The section of the road is extraordinarily light, being no more than one sixteenth of an inch thick in the web; in spite of this, no trouble has ever been experienced with it. Two rows of five sixteenths of an inch rollers are employed for the big end, an outer race being pressed into the rod. The bore and stroke remain as before, 74 x 81 mm, 349c.c.

Top:   aluminium piston in the o.h.v. engine.
Left:   neat external valve lifter cam.
Right: the roller bearing in the big end of the
          light connecting rod is carried in a
          pressed-in bush.

Plain splash lubrication is relied upon for the bronze main bearings. On the driving side of the crankshaft a standard spring loaded cam type of shock absorber is fitted, transmission following standard A.J.S. lines through a close ratio gearbox, the ratios supplied being 5 to 1, 6.1 to 1, and 9.4 to 1.

For the benefit of sporting riders who desire to enter speed events and hill climbs, the rear mudguard and carrier have been made in quickly detachable form. There is also a stay from the saddle tube to the rear of the cylinder head.

This overhead valve model does not replace the standard side0-by-side valve machine, which is still listed as before, but with 650 x 65 mm tyres, modified handle bars and a much improved front mudguard which, in addition to being splayed at the rear, is formed with mud-trapping channels round the edge of the valences, and a shield at the front to prevent the ingress of air, which usually blows back mud spray onto the rider. An aluminium piston is also fitted to this model.

Such features as the adjustable foot rests, neat knee grips fixed direct to the tank, the spring loaded hand oil pump, are of course retained, and the machine is finished as well as ever.

Modifications to the 799c.c. passenger outfit consists of aluminium pistons, and a similar but enlarged type of mudguard to that employed on the standard 349c.c. model, the fitting of a Lucas Magdyno and lamps are standard, and the arrangement of the change speed lever and switchbox further forward on the tank, so that they clear the rider’s knee with comfort.

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