Precision Motorcycle Engines

Today in Motorcycle History

Precision 350cc 1919

Precision 1919 350cc

Left side of the new Precision motor cycle, which embodies many new and practicable features.

Rear Wheel Mounts

The rear fork end and sliding wheel carrier, by which the wheel may be removed without disturbing the distance between the chain sprocket centres.

Front Brake

The Precision front brake. A Ferodo-lined band contracts upon a 10in drum.

Precision 1919 350cc Engine

Right side of the engine unit, showing the kick-starting device working on the engine shaft and the pedal which operates the countershaft brake.

Precision 1919 350cc Fork Spring

(Left) Although arranged V fashion, the front spring Is in effect what is known as a cantilever suspension. Note the roller levers and front brake operating rod.
(Right) A rear view of the handle-bar and forward end of the pressed steel tank.

350cc Frame Section

For the especial benefit of The Motor Cycle, a frame was cut through the centre to reveal the construction. The above is an " untouched " photograph, and shows the perfect joints made by welding.

Precision 1919 350cc, Front

Front view of the machine. Note the roller lever system (or operating the compression release and front brake and the short lever for changing gear


A perspective view of the gear unit, showing the segments which are expanded against the inner races of the drums connected to the two driven sprockets. The centre drum is the countershaft brake.


An Unconventional Solo Machine, embodying 330 c.c. Two-stroke Engine, with Gear Unit Integral, Rear Springing, and Pressed Steel Tank forming part of the Frame.

ALTHOUGH we have known for some time that Messrs. F. E. Baker, Ltd., the makers of Precision engines, were experimenting with a novel design of complete motor cycle, we were not permitted to record this interesting fact until our issue of October 2nd. We may say we have watched the development of this design from its earliest stages, and are consequently in a position fuHy to appreciate the general idea behind the design.

Mr. F. E. Baker has often told us that he aspired to produce a motor cycle meeting his own requirements, which, he contended, would also find appreciation with a large number of potential purchasers who desired most of the comforts of a car combined with the undoubted advantages of the single track machine.

Mr. Baker has always been a motor cycle enthusiast, and for. several years he has told us that, provided he could obtain the same comfort, as regards insulation from road shocks, as he had in a large car, he preferred the motor bicycle.

This first requirement meant a spring frame, which, taken in conjunction with other points considered to be essentials, was not quite so easy to design as its simplicity might imply.


Then other considerations were inspired by the road experiences of Mr. Baker and certain members of his staff on machines of all types. To produce a machine obviating the disadvantages and causes of irritation inherent with pre-war types was to be the aim. This, of course, was an engineer's ideal, but with these ideas on the one hand and manufacturing problems on the other, a very good compromise has been arrived at which is a sound commercial proposition.

The new Precision motor cycle, while retaining the general outlines of the conventional machine, differs from it at almost every point, the main aim being to secure the essentials by the least number of units, with an entire absence of parts attached to the frame by clips. How far the designers have succeeded in this the reader will be able to judge from the illustrations and details of the machine which we are now able to give.

The Frame-tank Unit.

In the first place, the tank and mud-guards form parts of the frame construction, and each is thus made to perform a double function. The tank is formed of two steel pressings, welded together, and takes the place of the two top girder tubes and detachable tank of the conventional machine, while the round section mudguards are made sufficiently strong to act as one of the sides in each of the triangulated wheel frames. While this is not the only instance of a tank being made integral with the frame, it is the first time steel pressings have been used in this manner. In building the frame, the head and front diagonal tubes are first welded together and then, with the seat tube, are enclosed by the two pressings forming the tank. The edges of the latter are machined before welding. For our especial benefit, Messrs. Baker had a stock frame cut through the centre in order that we might see the perfect joint so made, and we are able to give a photograph of these sections. It will be seen that such construction must be exceptionally strong without being unduly heavy. Two engine carrier lugs, another carrying the rear spring bracket, and the filler, are then brazed in their respective positions, and the main part of the frame is complete. The detail construction of the head and its bearings will be seen from the illustration on another page, the cups are pressed into position, but are renewable. The seat pillar is fixed by means of an expanding cone, similar to the almost general practice of fixing the handle-bars on first-class pedal cycles.

The present-day machine has frequently been criticised owing to cycle practice having been too slavishly followed. It is noteworthy that in the machine under review two points following cycle practice are embodied, which have either not been copied at all or copied but seldom in motor cycle construction. The first of these is the expanding cone fixing, just mentioned, and the second, the roller lever actuation of brake levers. These are the only points where the Precision follows previous cycle practice, and especially in the latter direction is the departure of interest, as it obviates the sharp bends usually present with cable front brake and compression release controls. Carried on a sleeve oscillating on the left hand lever, which operates the compression release, is another lever at right angles and fitted with a ball top. This actuates the gear change, and is sufficiently strong to be used for steering purposes while changing gear - a point which may be appreciated by the nervous solo rider.

The Springing System.

Of the many springing systems reviewed in The Motor Cycle, the Precision design is one of the most simple. Both wheels are carried in triangulated frames, of which a portion of the mudguard in each case forms one side. The method of construction is good, and quite up to the work, perfectly rigid members being obtained. At one corner of the triangle the members are pivoted to the frame, the hub being at the second point, and the end of the spring at the third.

In the case of the front wheel springing, the wheel frame (or front fork) is pivoted to the steering tube by means of a long bracket fitted with ball bearings of the cup and cone variety. Immediately above this "joint," and forming part of the steering column, is the support for a V-shaped spring-saddle, to which the inner ends of the two-leaf springs are fixed by means of clips. This saddle is free to oscillate, and therefore gives the springs the same effect as would obtain were a single half-elliptic "cantilever" spring used as in the rear springing.

By this method the advantages of a long spring are secured in a compact form, and, in addition, the load is so distributed that heavy anchorages are not necessary. By an examination of the photograph, the reader will appreciate that, if a single longitudinal spring were used, the saddle would have to be fixed rigidly, and this would have to be extremely strong in order to withstand the strains put upon it. The end of the two quarter-elliptic springs are shared to the top of the steering tube and to the bridge on the mudguard, the shackle bolts being fitted with lubricators.

At the rear the springing is similarly arranged, but here a half-elliptic "cantilever " spring is used. The spring-saddle in this case pivots on a fixed member attached to the seat tube, and the lower and longer portion of the spring is shackled to the frame.

It will be seen that, apart from the shackles, there is only one oscillating paint in each wheel frame, and as these are large bearings of ample dimensions, lateral rigidity is well provided. These bearings are capable of adjustment, but as the movement is so slight, and not to be compared in any way with the conditions prevailing in the case of a. wheel hub, such adjustment should not be necessary very frequently.

The wheels fitted are 25 x 2 1/4 in., and both may be dropped out of the fork ends in a vertical direction. The rear fork end particularly is worthy of mention, as the chain adjustment is entirely independent of the wheel fixing. This means that when the wheel is removed for any purpose it may be replaced exactly in the position it occupied before, the chain adjustment being undisturbed. This is obtained by the hub spindle being fixed in plates which slide on the fork ends, and are adjustable by draw bolts. The plates are secured by two bolts on each side, which are slackened off when chain adjustment is made.

Engine-gear Unit.

The 350 c.c. two-stroke Precision, which we described in our issue for May 29th this year, is more or less well known, and forms the main element of the power unit. This engine, it will be recalled, has a bore and stroke of 74 mm. and 81 mm. respectively, the capacity being 350 c.c, which makes it one of the largest single-cylinder two-strokes made at the present time.

In this engine there are several notable features, including automatic lubrication from a sump contained in the crank case casting, detachable head, and roller bearing big end. The new engine, however, embodies the countershaft within the base chamber, the transmission between the two being by chain on either side of the crank case proper, -which is enclosed in the main casting. Chain adjustment is effected by two jockey sprockets, the bearings for which are mounted eccentrically and are accessible from the outside of the case, the flywheel having an aperture through which a special key can be inserted to adjust the sprocket.

The two chains drive sprockets which are free to revolve on ball bearings on the countershaft, which in turn carries, on, the outside of the case, the small driving sprocket of the final transmission. Also fixed to the countershaft is the drum of the band brake, which takes the place of the more usual rear wheel brake. The band is of steel, anchored to a lug approximately on the centre line of the" unit, and is actuated by a lever on a shaft to which, on the outside, is fixed a long pedal lever. In principle, the gear is reminiscent of the old De Dion clutch, but is quite new in motor cycle practice, and probably unknown to the majority of motor cyclists. In principle, the gear is of the expanding clutch type, so that both gears are equally direct. The driven sprockets each carry a drum, against the inner faces of which are engaged, as desired, the clutch members revolving with the countershaft.

Expanding Clutches.

These clutches, consisting of two segments, are expanded by means of a rack running through the centre of the spindle and engaging with toothed wheels, operating right and left-hand "jack screws." Provision is made for independent clutch adjustment, for which purpose the rack, which takes the form of a thread, is made in two parts, so that either may be rotated in order to give a worm and worm-wheel effect to the toothed wheels, and so open or close the clutch blocks. To prevent unintentional rotation of the rack, spring-operating plungers engage with flats on the side of the racks, and ensure the flats on each being in line, the threads of the two so mating that the joint between the two may pass either toothed wheel. The racks are held together by the actuating rod, which passes through the centre of both, balls being interposed between the nut on each end of the rod, and the outer end of each rack to form journal bearings to carry the rod and thrust bearings for the endwise movements. To effect adjustment, the parts are so proportioned that when either clutch is in operation, its" rack protrudes from the spindle to allow of turning it by a suitable tool, and the amount it protrudes is an indication of the correctness of the adjustment.

The top gear of the machine at present is 4 to 1, on which ratio it is claimed that 50 m.p.h. has been attained, but probably a 6 to 1 ratio will be standardised.

As previously mentioned, the gear change lever is on the handle-bar. The actuation of this is quite simple, and merely involves slightly turning a diagonally disposed shaft extending from the centre of the countershaft to a support behind the lower ball race of the head. On the lower end is a small crank, which is connected to a toggle operating the rack engaging the worm wheels in the segment carriers.

The final transmission is by chain, which in finished production models will be protected by a guard. The difference between the respective positions of the countershaft and the pivoting point of the wheel frame is perhaps a little greater than is to be desired by the fastidious engineer, but is no more than is the case with the majority of spring frame designs, the ideal, of course, being to have the driving sprocket and the fulcrum of the wheel frame on the same centre.

Other Notable Features.

Several other features are worthy of note, including the three lever fitting for operating the throttle, air, and magneto. For these three controls Bowden wires are used, the cables for which will be enclosed in a single tube.

Aluminium footboards of generous dimensions, a metal tool box with wooden lid integral with the rear mudguard, a Brooks B210 leaf spring saddle, and a large undershield form part of the equipment. The tank capacity is two gallons.

As regards its performance on the road, we await with interest an extended test, but from our five miles ride on it over rough and pot-holed roads we were very favourably impressed by its running and the efficiency of its springing. To those who have not ridden a spring frame mount, undoubtedly the extra comfort provided will come as a revelation and a new sensation in road transport. Probably it has only been the difficulty of providing a frame laterally rigid that has deterred many makers from abandoning the rigid frame. These difficulties appear to have been overcome in the Precision, and if, in the opinion of conservative motor cyclists, its appearance is not so graceful as perhaps it might be made, it must be remembered that designers who too closely study conventionality restrict their scope in bringing motor cycle design to finality.

The Motor Cycle October 16th, 1919. pp 418, 420, 421