A reciprocating cylinder is rigidly fixed to the loco frame and the piston travels to and fro inside it, with a neoprene ring on it to minimise wear and keep it steam tight, and the piston rod sliding in the rear end through a gland rendered steam tight with either a neoprene ring or graphite yarn packing. A connecting rod is necessary to articulate the reciprocating piston rod to the rotating crank on the wheel. Distribution of live steam and evacuation of exhaust steam in the cylinder is effected by a valve moving in an attached valve chest, its movements being controlled by a valve gear actuated by either an eccentric or a return crank on the driving axle. The valve may be either a flat slide valve or a piston valve. Locos built by model engineers for our small scales usually have slide valves because piston valves are more expensive to make on small hand-crafted runs. To be steam tight a piston valve must, if not fitted with rings, be machined to exceedingly fine tolerances, and wear soon leads to steam leaks, while if grooves are cut on a small diameter piston valve and then packed with graphite yarn, there is a deal of frictional resistance, possibly enough to ruin the en-gine's performance. In any case with yarn or with neoprene rings, the ring is liable to damage as it passes the steam ports. With a slide valve, the face of the valve remains steam tight if wear takes place, and frictional resistance is minimal. Piston valves can be designed to reverse by reversing the steam flow, a characteristic which makes them adaptable to remote control. Slide valves cannot be reversed in this way; they must be moved from one end to the other in the valve chest, and this can only be done through an adjustable valve gear. The most usual gear in, our scales is the slip eccentric, which runs loose on the driving axle and is pushed into position for forward or reverse valve travel by a stop collar having two driving faces, that is fixed beside it to the axle. To bring the appropriate face of the stop collar into play, the loco is pushed by hand a short distance in the required direction. This arrangement is not amenable to remote control, and if this is desired, a full prototypical valve gear is essential, whereby the eccentric is fixed on its axle, and vibrates a die block, in which the end of the valve rod can be slid from end to end by means of a remotely controlled lever. Elaborate valve gears can be expensive, especially if the parts have to be hardened to minimise wear.

Lubrication is something that especially concerns the steam operator, for if it is neglected there is no mains electricity to drive the engine on its way remorselessly regard-less of its squeaking. Both long life and everyday performance derive from a ritual administration of the oil can. The oil to use is one formulated by one of the major oil companies specifically for steam engines, say Shell Valvata or Castrol Cresta, or one of the nameless oils (possibly the same, repack-ed!) supplied retail by the Model Engineering Trade.

While the conscientious driver regularly visits with his oil can all the visible moving parts, his locomotive still stands in nee o an apparatus to refresh those parts that ordinary oil cans cannot reach, to wit, the interiors of the cylinders and valve chests. Every hard working engine needs an automatic lubrica-tor, either mechanical or hydrostatic. The former type is the more reliable, as a ram driven by an eccentric on one of the axles sends a shot of oil into the system with

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