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Aviation History
1926
1926 - 0822.PDF
NOVEMBER 11, 1926 engine, the Mark II is now available. Incidentally, it shouldbe pointed out that the newer engine passed its Air Ministry type tests some time ago, so that machines fitted with it arceligible for their airworthiness certificate. A detailed general description of the Mark II " Cirrus " isscarcely necessary in view of the fact that it is, with a few exceptions, identical with the older model, and, in fact, willfit^the standard engine bearers of the Mark I, so that for use is of the metal-to-metal type. A small change has been madein the shape of the cylinder-head casting, which results in a much simpler job. The rocker arm brackets, which were castintegrally with the head in the older model, are now a separate piece in the form of a Duralumin forging, bolted to the head.A free air passage between the valve domes is thus obtained. The two valves in each cylinder are placed in a very " clean "combustion chamber, as the view of the inside of a cvlinder [" FLIGHT " Copyright Sketches THE "CIRRUS MARK II ": Some constructional details. In 1 are shown details of the mounting of the rocker spindle, the adjustments of the push rods, and the arrangement for Tecalemit lubrication of the rockers, &c. 2 shows a piston with piston rings, gudgeon pin and circlips. Details of the valves are shown in 3, the cupped valve being the inlet. The flexible magneto drive is illustrated in 4, while 5 shows the somewhat unusual induction manifold. This is exhaust heated, and the two outer branches are flexibly connected to the fixed centre portion by short lengths of rubber pipe secured by " Jubilee " worm-drive clips. in machines hitherto fitted with the original " Cirrus " thesubstitution of the new model is a very simple matter indeed. The four air-cooled cylinders are generally similar to thoseof the older engine, but are slightly larger to the extent that their bore has been increased from 105 mm. to 110 mm. Thestroke remains as before, 130 mm. The cylinder barrels, with their fins, are of cast iron, while the cylinder heads areof aluminium, and are held down by four long bolts to the top of the crankcase. The joint between the head and the cylinder 730 head will show. They take their seats, not on steel valveseats, as in the Mark I, but on phosphor-bronze seats expanded into place in the head casting and held in position by theexpanding only, no other form of locking of the seats being provided. The valves are operated by push rods from the camshaftlocated inside the crankcase. Duralumin tubes are used for the push rods, and two adjustments are provided in eachpush rod, one, set at the top and another a short distance
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