The camshaft for the exhaust valves (101, Figure 6), was chain driven from the crankshaft and was carried along the bottom of the crankcase in three babbit-lined bearings in bearing boxes or lugs cast integral with the case. Both the driving chain and the sprockets were standard bicycle parts, and a number of bicycle thread standards and other items of bicycle practice were incorporated in several places in the engine, easing their construction task. The shaft itself, of mild carbon steel, was hollow and on each side of an end bearing sweated-on washers provided shoulders to locate it longitudinally. Its location adjacent to the valves, with the cam operating directly on the rocker arm, eliminated push rods and attendant parts, a major economy. The cams were machined as separate parts and then sweated onto the shaft. Their shape shows the principal concern in the design to have been obtaining maximum valve capacity--that is, a quite rapid opening with a long dwell. This apparent desire to get rid of the exhaust gas quickly is manifested again in the alacrity with which they adopted a piston-controlled exhaust port immediately they had really mastered flight and were contemplating more powerful and more durable engines. This maximum-capacity theory of valve operation, with its neglect of acceleration forces and seating velocities, may well have been at least partially if not largely the cause of their exhaust-valve troubles and the seemingly disproportionate amount of development they devoted to this part, as reported by Chenoweth, although it is also true that the exhaust valve continued to present a problem in the aircraft piston engine for a great many years after, even with the most scientific of cam designs. The rocker arm (102, Figure 6) is probably the best example of a small part which met all of their many specific requirements with an extreme of simplicity. It consisted of two identical side pieces, or walls, of sheet steel shaped to the desired side contour of the assembly, in which were drilled three holes, one in each end, to carry the roller axles, and the third in the approximate middle for the rocker axle shaft proper. This consisted of a piece of solid rod positioned by cotter pins in each end outside the side walls (see Figure 5). The assembly was made by riveting over the ends of the roller axles so that the walls were held tightly against the shoulders on the axles, thus providing the correct clearance for the rollers. The construction was so light and serviceable that it was essentially carried over to the last engine the Wrights ever built.