A 4 ¼ h.p. Four-stroke Single-cylinder. Unit Construction. Pump Lubrication.
THE 350 c.c. two-stroke Beardmore-Precision being now firmly established on the market, Mr. F. E. Baker has once again given his mind to the production of the big single fourstroke. In spite of the fact that the pre-war 4 ¼ h.p. Precision was sold overseas in greater quantities than at home, there are still many admirers of that popular engine in this country who have been looking forward to the appearance of a post-war Precision four-stroke.
Founded on the main essentials of the old "4 1/4," the new engine is a combination of the most up-to-date practice with the results of years of practical experience.
To begin with, the engine is lubricated automatically by a mechanical pump ; and, though a simple supply adjuster is provided, once this has been set the driver is relieved of all further worries. Again, the engine, gear box, and magneto form a single unit, the crank case and gear box being cast together. At the same time, the two units are separated by the walls in such a manner that, but for the constructional advantages, they might as well be two entirely separate units. The mechanism of each is inspected through separate end plates, and no particles of foreign matter from one unit can affect the working of the other.
597 c.c. Cylinder Capacity.
A cylinder of 89x96 mm. bore and stroke is bolted to the crank chamber in the normal way, but the side-by- side valves are mounted in a detachable cylinder head, which has radially disposed cooling fins, and is held down by four set pins.
Within the cylinder lies a slightly waisted cast iron piston with two top rings and one in the skirt which acts as a scraper ; double row roller bearings are employed for the big end. Two ball bearings support the crankshaft on the drive side and one on the timing side, and the flywheels are notable for their large diameter and the fact that they are forged solid with their shafts, leaving only the crank pin to be assembled. A separate gear wheel, cam, and light rocker are employed to operate each of the adjustable tappets, and a subsidiary exhaust cam is brought into action when required to act as a decompressor.
Directly below the crankshaft pinion lies a bronze wheel, having a 3 to 1 reduction, which drives a rotary vane type pump, and delivers oil from a sump (formed by a chamber below the gear box and an enlargement of the timing cover) to a regulating chamber which lies between the tappet guides. A notched quadrant provides several adjustments for regulating the oil flow through a simple needle valve, and oil is delivered from the regulating chamber to the crank case through a ball valve. In addition, a pedal on the exterior of the crank case operates a disc valve, by means of which the crank case can be flushed with oil for racing purposes or in case of emergency. A special crank case breather returns the majority of the otherwise waste oil to the sump, but a small quantity is fed to the primary chain.
So short are the centres of the primary drive that no adjustment is provided, since when the chain is stretched sufficiently to cause bad running it should be replaced. The gear box housing is arranged so as to accommodate standard Sturmey-Archer gears, shafts, and clutch, though a modified starter and change-speed quadrant are fitted. The primary chain is almost entirely enclosed by a cast aluminium case, and special provision is made for trapping mud thrown off by the rear chain.
This most interesting unit is housed in a frame almost exactly similar to that already popularised on the spring frame Beardmore-Precision two-stroke. The same type of steel tank forms the main member of the frame. Similar front and rear springing is employed, and no casts lugs are incorporated, all joints and lugs being made of steel pressings welded together where necessary. Both brakes are of the contracting band type - that on the rear wheel being wide and particularly powerful.
Detail modifications include a pressing to replace the tubular member between the front spindle and the leaf spring, cable guides and pump clips brazed to the tank, and a neat purple panel outlined with blue.
It will be remembered that the mudguards form part of the frame, and that the combined tool-box and carrier are part of the rear guard. Very sensible aluminium footboards with, long upward extensions are part of the equipment, and 700x80 mm. or 26x3in. light car tyres are fitted.
It should be emphasised that the engine unit is not a modified 500 c.c. engine, but is designed throughout for its ultimate purpose, and that the complete machine is intended as a competitor to the low-priced light car. As a sidecar outfit, the price of this very fine machine will be £135, while the solo price will be £105.
The Sports two-stroke will, of course, be continued, and will sell at £75: and a machine similar in all details, but fitted with a 350 c.c. Barr and Stroud sleeve-valve engine (described in The Motor Cycle of July 21st), will be listed at £85.
Sources: The MotorCycle November 10th, 1921.