The design exemplifies a type of tank engine introduced on British Railways in 1951 and is one recommended for modelling in 2½-inch gauge or larger if desired. It is suitable for heavy duty on an outdoor model railway system. The weight on all six-coupled wheels should not be less than three-quarters of the total weight of the engine in working order. The tractive effort is 10 lb., and the engine is capable of hauling a load of 250 lb. on a level track.
The front and rear ends of the engine are guided by a swivelling bogie and pony truck respectively, and therefore the locomotive rides very well indeed on lines with sharp curves. The minimum radius recommended for 2½-inch gauge is 10 feet (3 metres).
Cylinders. 5/8 -inch bore by 1 1/8-inch stroke. Two outside cylinders only inclined on a slope of 1 in 16 with the valve chests on top. Piston-valve or slide-valve cylinders may be fitted.
Valve Gear. Outside Walschaerts' designed to give a maximum cut-off of 85% of the stroke.
Slide Valves. Steam port opening 0.07 by ¼ inch wide. Exhaust port opening 0.14 by ¼ inch wide. Thickness of port bar 0.08 inch. Lap of valve 0.046 inch. Total valve travel 0.232 inch.
Coupled Wheels. 3-inch diameter on the tread and of a good close-grained cast iron. Wheels to be an even press fit on ground steel axles and keyed. The right-hand crank-pins to be a quartet turn in advance of the left-hand crank-pin (90°). The axles to be 3/8-inch diameter and carried in gun-metal axle-boxes on underhung coil springs.
Bogie. As the trailing end of the engine overhangs the track a considerable amount when negotiating a sharp curve, the slot in the bogie stretcher must be wide enough to allow side play. of the central pin. Allow for a total movement of 3/8-inch. The bogie to be, equalized and sprung. Wheels 1 19/32-inch diameter.
Pony Truck. The wheels are 1 19/32-inch diameter! The truck is sprung and pivoted at a point 3¼-inches behind the axle.
Boiler. Locomotive type with narrow firebox. It may be designed and tested for a working pressure of 65 lb. per square inch on the gauge.
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