Rex-Acme Motorcycles

Today in Motorcycle History

Rex 1919 Motorcycles UK

Rex 1919 550cc Frame & Engine Details

The steering head, showing the method of lubricating the ball races, and the cone fixing for the handle-bar.
The drilled connecting rod and aluminium piston. The latter is fitted with three 1/4 in. rings, two of which are stepped and have scraper grooves.

Rex 1919 550cc Crankcase

The crank case and oil sump. The oil sump filler and level indicator needle are shown on the left.

Rex 1919 550cc Single

The new Rex, which will be known as Model 77, primarily designed for sidecar work.

Rex 1919 550cc Timing Chest

Timing gear of the Rex engine, showing the decompressor mechanism.


84 1/2 mm. Bore, 98 mm. Stroke, 550 c.c. Capacity, Mechanical Lubrication.

ENTIRELY new models have been few and far between, many 1919 models being war-time designs improved in detail. So far as singles are concerned, it is about five years since an entirely new design was introduced. Therefore, the Rex 4 h.p. sidecar single undoubtedly will create a great deal of attention on this score alone. On two other counts this machine is particularly interesting. First, it is the latest model of a firm which has been associated with the motor cycle industry since its earliest days; and, secondly, the design is interesting in itself, both as regards the engine and the machine generally.

The latest Rex is essentially a sidecar machine. It has been designed throughout for passenger work, and is, consequently, heavier than most machines of the same capacity which are designed as dual purpose mounts. The requirements of the Overseas markets, too, have been borne in mind, hence the American type rims and 28 x 2 1/2in. tyres.

The Engine.

In design thee power unit departs from the average single-cylinder motor cycle engine in at least one important point, i.e., the lubrication system. The crank case is in three main parts, two sides, and a base. The latter forms the bottom of the crank case, and also embodies a large oil sump extending forward to form the platform for the magneto.

On the left side of this sump is the oil filler, which has a screw needle for ascertaining the amount of oil in the sump.

From the sump the oil is circulated by a small gear pump on the right side of the unit, through an adjustable sight-feed on the tank, to a union on the crank case, which communicates with a circular channel, from which it overflows into a second channel, of which the base of the cylinder barrel forms the inner wall. In the latter are several holes for the entrance of the oil into the engine proper, from which the piston also takes its allowance.

The residue is constantly dripping into the base of the crank case, where it is maintained at a certain level for splash lubrication, after which it drains back to the sump via three holes in each side wall. These holes also serve automatically to provide more oil when the machine is going uphill, as, when the machine assumes certain angles, these overflow ports one by one go out of operation, and in consequence a greater depth of oil is accumulated.

In addition to the mechanical oiling system, there is an auxiliary hand pump which can be used to give the engine an extra charge of oil from the tank if or as required. The pump is on the side of the tank, and placed much further to the rear than is usual. Thus the Rex engine has two, distinct oiling systems.

The cylinder has a bore and stroke of 84 1/2 mm, and 98 mm. respectively, the capacity being 550 c.c. There appears to be ample radiating surface on the fins, which, along the side of the valve pockets, are cast straight instead of following a more or less circular contour; and between the valve pockets and the cylinder there is a clear air passage.

Side-by-side valves are used, operated by a single cam - in fact, one cam serves for inlet and exhaust valves and the decompressor.

The Decompressor.

The latter mechanism consists of a finger carried on a sleeve, which is mounted eccentrically on a shaft, to which the operating lever is fixed. Connected to the sleeve is an arm extending to an auxiliary rocker integral with the exhaust valve rocker proper, but is not connected directly to it. The auxiliary rocker has a stud fitting in a slot in the arm. Therefore, when the decompressor lever is so moved that the finger gets into the path of the cam, it partly rotates the sleeve and brings the stud to the end of the slot and so raises the exhaust valve during a portion of the compression stroke.

A single shaft, placed centrally, is used to carry the rockers, and the cam wheel is off the centre line. The other gear wheel in the timing case is for the purpose of driving the oil pump. There is a worm on the shaft of this latter gear, which drives a phosphor bronze wheel on the short shaft, rotating the gear of the pump. This latter is a neat little device, consisting of two spur wheels in a small brass case, which can be completely disconnected by removing the union and six screws, without disturbing the timing case coyer or any other part of the engine.

Aluminium Piston.

It is extremely interesting to note that the makers of the new Rex are pinning their faith to an aluminium alloy piston. This is the subject of one of the several patents about the machine, and weighs threequarters of a pound, or about half the weight of a cast iron piston of the same size. As will be seen from one of the accompanying drawings, the piston is of the lantern type, while closer examination will show that two of the three rings are of unconventional section.

The designer has aimed at obviating direct contact between the aluminium piston and the cylinder wall, and this is attained by the L section rings shown. Neither the top nor the bottom of the piston can touch the cylinder walls, as each end of the piston is cut away and the space filled by the ring. A groove is cut in these rings to distribute the oil along the path of the piston and to remove the surplus. A third ring below the top ring is fitted, and like the others, it is 1/4 in. wide.

The gudgeon pin works directly in the aluminium bosses of the piston, the small end being split and gripping the pin by means of a nut and bolt. A large bearing surface is obtained in this way, and we may say we have examined a piston which has done over 2,000 miles, and no appreciable signs of wear were apparent.

A double row roller bearing is used in the big end of the connecting rod, which is particularly light. The main shafts are supported on large ball bearings with an extra bearing of the roller type on the driving side.

The wheels are of the quick detachable type with knock-out spindles, and are interchangeable, the pulley wheel, which is carried on a separate ball bearing, remaining in the frame. As will be seen from the drawing of the head, the lower ball race has a proper oil channel leading directly to it from a large lubricator in front of the head, while the top ball race is lubricated through oilways drilled at an angle from four holes, which are used also for adjusting the head. These holes are covered by a sliding ring.

Adjustable Handle-bars.

An adjustable handle-bar is another feature of the design ; the bar, being gripped by a split lug, may be raised or lowered as desired by slackening two nuts. The other part of this lug fits on the steering head over a long tapered member having a key, and is held in position by a large "hut" having an extension for accommodating the lamp bracket. Brampton Bi-flex forks are fitted, with a good size mudguard and a front wheel stand. The front brakes are of the conventional type, but the wheel is rendered accessible by the brake shoes being so made that they can be swung round by loosening the two nuts which secure them to the stirrup.

The frame design departs from past and present practice. The top tube slopes to the rear but towards the rear end it is bent. From this tube the cantilever member of the saddle suspension is pivoted. The four sidecar lugs are integral with the frame, and the carrier member embodies a large metal tool box, which forms part of the rear guard. By removing four nuts this unit, consisting of carrier, tool box, and rear portion of the guard, may readily be removed.

Parallel Motion Rear Brake.

Rex 1919 550cc Diagrams

1. The brake pedal is well supported on a long bearing, which has a large lubricator.
2. Lubricator to supply oil to the ball bearing supporting the belt rim.
3. The rear brake, which has a parallel motion, The brake spring is enclosed.
4. The handle-bar lug, which permits the bar to be adjusted as to height.

The rear brake has a parallel motion, and the fibre pad works inside the V of the belt rim. This brake is operated by a pedal, and is the only control at the driver's feet, all others being hand-operated.

On the left handle-bar there are the exhaust valve lifter and clutch levers, and on the right the front brake and carburetter controls. The magneto advance and retard lever is, perhaps, the largest we have seen on a motor cycle. This is located on the left side of the tank.

The finish of the machine is in black enamel, with very few plated parts, the handle-bars being finished by a new process, which renders the metal a dull black finish, somewhat similar to what is known as "gun finish." The tank is black and gold, with a mauve panel.

A Sturmey-Archer three-speed gear box gives ratios of 5.2, 8.3, and 13.6 to 1.

The proposed price of the machine without sidecar is £82 10s.

The Motor Cycle, April 17th 1919. p385