Precision Motorcycle Engines

Today in Motorcycle History

Precision Engines 1914-1915

The 4h.p. Twin Precision.

THE 4 h.p. twin Precision is described by the makers as a lightweight twin, as it is claimed that, on account of its flexibility and smooth running, it does not require a heavy frame or fittings, while the weight of the engine itself is only 57 lb. It is, of course, not to be imagined that a machine fitted with this engine should be a lightweight proper, but that it should be very light for the power. The design follows standard Precision lines, but there are several interesting features peculiar to the engine. The bore and stroke are 60 and 88 mm.

A new inlet pipe is fitted which will be standardised on all Precision twins, and the manufacturers are to be congratulated on taking so much interest in a much neglected part. The pipe is somewhat flattened to permit of an easy passage between the valve chests, and flange joints are utilised to make a sound connection. To prevent air leaks in the pipe the flanges are bolted up tight to the cylinder, and a sliding joint is formed by two separate parts of the pipe, while in addition a light stuffing gland is formed by a screwed ring packed with special material.

The internal exhaust lifter is another noteworthy point, for a spring tappet projects upwards from the top of the timing case, while the lower end rests upon extension arms of the exhaust valve rockers. Thus, on depressing the tappet by means of the usual handle-bar control, both exhaust valves are lifted evenly, a point which... helps easy starting and renders the valve gear quieter when coasting down hill. Two cams only are used, one for both valves on each cylinder.

The Motor Cycle, July 16th, 1914.


IN the August edition of The Motor Cycle Overseas supplement we dealt fully with the Peerless Precision twin. At that time the makers were not prepared for the home market, but more recently it has been decided to supply the engine in this country under certain circumstances.

We may truthfully say that the engine is probably the finest production which has left the Precision works, and this is a high testimony to its design and manufacture. The engine has a capacity of 602 c.c. and is of the long stroke type (56 X 88 mm). Externally, the most striking features are the outside flywheel, side-by-side valves, and the domed cylinder casting. The usual Precision square radiating fins are replaced by the more conventional circular type. The valve gear is very well carried out, and neat adjustable tappets are employed The exhaust lifter is operated by a fifth or central tappet, which can be depressed by a rocker, refreshingly stout in design. The construction of the big end is unusual, for a double row of roller bearings is carried on the crank pin.

A steel sleeve in the forked connecting rod forms the housing, while the central connecting rod oscillates on the outside of the sleeve. The inlet pipe and exhaust pipe are held by flanges, the former being fitted with a neat expansion joint. It may be mentioned that the cylinders are interchangeable.

The overhead valve twin-cylinder engine is not quite such a novelty, as its forerunners made their appearance in the Tourist Trophy Races, and have since proved their worth in daily use and competitions. The engine is in every way suited for sporting mounts: it is also an excellent touring engine, being flexible and smooth running as well as being capable of high speeds.

The chief change in the Precision Junior lies in the new cylinder and valve gear The bore has been increased to 65 mm., the stroke remaining at 50. Both valves are now placed vertically behind the cylinder. The rest of the unit has been modified but slightly, and, in spite of the increased power, a considerable degree of silence has been attained.

It is to be understood that these models are supplementary to, and, with the exception of the Junior, do not replace present types. The big single is, of course, still a popular type of engine.

The Motor Cycle, 1914