A Newly-designed 270 c.c. Two-stroke with Unit Assembly of Engine, Gear Box, and Magneto.
THE 2 1/2 h.p. Metro-Tyler two-stroke has been completely redesigned for 1920, and but few of the original details, which made the early machines of the same make so successful in such a short period, have been retained.
It will be manufactured in one standard model only, and this ha.? a surprisingly neat and compact appearance, since the engine, gear box, and magneto are in one complete unit, which is mounted . between two steel plates forming a housing.
Of the three-port two-stroke type, and having a bore and stroke of 70x70 mm. (270 c.c), the engine ha* a cast iron cylinder fitted with an inspection port over the transfer passage. Special , attention has been paid to ensure the adequate cooling of the combustion head, insomuch that the three top cooling fins are ? of aluminium, which rapidly disperses the heat generated. The cylinder head is machined both on the inside and outside. The crank case is ingeniously constructed, the design having been carefully considered in order to provide and maintain a high initial compression. The use of large diameter Skefko ball bearings necessitates some device to maintain compression within the crank case, and in this case the one evolved consists of spring diaphragms with bronze faces in contact with floating steel washers, which latter are held in position on either side of the crank case by the magneto and chain sprockets respectively. The diaphragms are held between two fibre washers, the outer one being of greater width than the inner one, thus distorting the diaphragm, and holding it tightly against the floating washer. Should wear take place, the washers -may be changed over, and will effectively take up the play. The crankshaft is a one-piece steel stamping and is provided with an integral balance weight. By accurately grinding the bevelled edges of the crankshaft webs and the rounded edges of the big end, the connecting rod may be passed over the shaft and placed in position to accommodate the rollers of the bearing. The big end of the connecting rod embodies the outer ring for the rollers, whilst the crank forms the inner member. This bearing has no cheek plates or rivets. The retention of the rollers in position is effected by means of a plate which is screwed to the corresponding area cut out of the crankshaft stamping.
The connecting rod is bushed at the small end, and the gudgeon pin is secured to the cast iron piston by means of a set screw and locking pin.
New ideas are noticeable in the design of the decompressor, which is housed in the front of the cylinder, and directly opposite the sparking plug. By removing these two parts a clear way is made across the top of the piston, which allows the removal of carbon deposit without dismantling the cylinder.
The construction of the decompressor. which releases directly into the exhaust pipe, is clearly shown in the illustration.
An ingenious system of lubrication is employed, the quantity of oil admitted to the engine being regulated by the adjustable drip on the Enots lubricator, whilst the supply is maintained by suction from the induction pipe.
A spiral oil groove is cut in the cylinder wall, one end o£ which is connected to the induction pipe, and the other to an oil lead from the lubricator.
A proportion of the oil which is sucked into the spiral groove lubricates the piston and gudgeon pin, which latter is drilled and grooved, and eventually passes through the induction port in the cylinder into the crank chamber; the remainder of the oil, however, is drawn into the crank case, and there lubricates the crankshaft ball and roller bearings. Should it be found necessary, an auxiliary hand pump is provided, whereby the supply of oil may be augmented. The two-speed gear box, which is completely concealed by and housed in the side plates, follows standard practice. Special attention, however, has been paid to lubrication and the method of engaging the dogs employed. The same oil as is used for engine lubrication is employed in the gear box, a large filler cap being located on top of the casting.
Efficient Gear Box Lubrication.
In addition to the oil holes drilled between the constant mesh gear teeth for lubrication of the bushes, the splined shaft has a left-hand spiral cut in the bearing end which leads to the straight oil channel cut in the splined portion.
The two gear ratios employed are 9.84 to 1 low gear, and 5.25 to 1 high gear.
In order that the dogs may be gently engaged, a control lever embodying a spring device is employed, which, regardless of the dog's position, may be moved around the quadrant fitted above the petrol tank.
The gear box may be moved along slotted holes drilled in the side plates for chain adjustment, whilst the C.A.V. magneto, which is secured to an aluminium bracket between the engine and gear box, may be adjusted in a similar manner.
Accessibility forms one of the most prominent features of this new machine, the complete power unit may be removed from the main frame in a few minutes, the chain cases are detachable for chain adjustment, and an inspection cover is provided for magneto adjustments.
The petrol tank, which is of very pleasing appearance, has a capacity of one and a half gallons, whilst the small oil compartment contains one quart.
The price of the machine, deliveries of which will commence early in February next, is not definitely fixed. It will, however, be somewhere in the vicinity of sixty-seven guineas.
The Motor Cycle, November 20th, 1919. p595