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The base is in two parts, split on the line of the shaft and bearings. The valve chest is on the back of the cylinder; I is the inlet opening and E is the exhaust; V is the valve stem. The gears are inside the case, as in Fig. 15; T is the ignition device. This particular type requires very little outside lubrication as the base, being tight, may be partially filled with, oil, which by the action of the crank, is splashed over the working parts, keeping them well oiled.

At P is the electric igniting device, and at c is the compression cock. The elbow A, leading to the inlet valve, is held in place by a clamp, and when removed exposes the two valves.

In a single cylinder engine, the valve chest may be in the most convenient position. Referring again to Fig. 13, the inlet and exhaust chambers may, instead of being on opposite sides of the cylinder, be placed alongside of each other on the same side, and may then be actuated by the same cam shaft, saving some complication and making the engine rather more compact. In multiple cylinder engines this is a very common arrangement although seldom used in single cylinder engines.

The mechanical operation of the valves and the fact that the impulses take place only on alternate revolutions, make the proper sequence of events possible for revolution in one direction only, with the usual types of mechanism. For this reason the four-cycle engine can be adjusted to run in one direction only unless by means of special valve gear. There are four-cycle engines built which may be run in either direction, but they are not common.

For starting the four-cycle engines, a handle in the flywheel rim may be used, as in the two-cycle, but the more common device is a removable crank fitting over the end of the shaft jnst in front of the flywheel and provided with some kind of rachet attachment to allow the engine to continue its rotation, leaving the handle stationary. This is possible, since the four-cycle engine luns in one direction only.