Fig. F-14–15 in Chapter V, shows the valve gear of an upright engine having the inlet and the exhaust valves located in pockets placed at one side of the cylinder. The inlet valve is operated by a valve rod that is actuated by the cam. The exhaust valve stem is raised and lowered, directly, through a cam on the same shaft. The method of driving the valves in this engine is practically standard for all vertical engines having the valves located in pockets. This system is used in a greater proportion of automobile engines. The opposed engine has the cylinders arranged on opposite side of the crank case, and makes an exceedingly well balanced and quiet running engine; as there is no point in the revolution where either the crank throws or connecting rods have an unequal angularity, or differ in velocity. While this type of two cylinder engine is common in automobile practice, it is not often met with in stationary work, the cam-box and the cam being directly in the center of the crank case. The opposed type of engine is particularly well adapted for aeroplane service as a steady, quiet running engine is an absolute necessity because of the frail construction of the aeroplane frame. (134) Cam Shaft Speeds. The valves of the gas engine are opened and closed by means of cams or eccentrics, that are geared to the crankshaft, and which also control the timing. As a four stroke cycle engine performs all of the events, or a complete cycle in two revolutions of the crankshaft, it is evident that the cam must go through the routine in one revolution or must revolve at =ONE-HALF OF THE CRANKSHAFT SPEED=. Therefore the cam gear ratio must be as one is to two, the smaller gear being placed on the crankshaft, the gears being known as the “half time gears.” As a two stroke cycle engine goes through the routine of events in every revolution, the cam-shaft must run at crankshaft speed so that the cam outline makes one revolution in the same time as the crank. The cam shaft speeds given here apply to all engines of the corresponding cycle no matter whether the valves are of the poppet, rotary or slide-sleeve types. (135) Valve Gear Troubles. The valve gear mechanism causes trouble principally through the wear of the various parts which results in a change in the valve timing, or in the lift of the valves. Loss of power, =MISFIRING=, and overheating are the result of such derangements. Often trouble is caused in reassembling the valve mechanism after the engine has been torn down for repairs, which trouble may generally be traced to incorrect gear meshing. The following list will give the principal defects due to the wear of the valve mechanism.