lv

r are the sentinels placed on guard at the entrance to and exit from each cylind

aid to be valveless because of the fact that the movement of the piston automatically regulates the flow of the exhaust and intake gases, but in this case the piston is in reality the valve. On the four-cycle motor, howe

tion and placed in the cylinder head. When in the former location, the valve is opened by an upward push on the rod to which it is attached at its center, while a valve placed in the cylinder head is forced down to allow the escape or entrance of the exhaust or intake gases. The

d push caused by the rotation of a cam and are returned to their closed position by means of the extension of a stiff spiral spring surrounding each valve stem. It is only the outer edg

when the valve is closed. On the majority of poppet valves the edge of the head and the seat against which it rests are beveled to an angle of approximately forty degrees in order to conform to the natural direct

ll closed position of the valve and that the operating mechanism for obtaining this lift is simple. Practically every poppet valve, therefore, is lifted by means of a cam, which is a thick, irregularly-shaped piece of steel mounted on a shaft known as the cam shaft. If the end of the va

this beyond the smallest diameter of the cam being the distance that the valve will be lifted when this point of the cam surface comes in contact with the stem or push rod. The broader, or more blunt, the nose of the cam, the longer will the valve remain open as the cam shaft is revolved, while the "slope" of the sides

f the push rod at all times against the periphery of the cam. This push rod, in some instances, is a small bar of special steel that slides in guides of long-wearing bearing alloy. The upper end of the push rod

lat disc slightly "offset" on the end of the push rod so that various points of its surface will come in contact with the periphery of the cam and the wear will be eve

cam shaft as those used to open the valves at the side. As the opening of a valve located in the head is downward, however, the motion produced by the cam on the push rod must be reversed in direction. This reversal of motion is obtained by means of a lever mounted at its center and placed in contact with the upper extremity of th

the valve stem to the push rod, and thence to the periphery of the cam when the latter is in a position to lift the valve. The push rod should not be forced tightly against the periphery of the cam when the valve is closed, however, for this would prevent perfect contact

use the opening of the valve to occur late and will reduce the distance that the stem is raised, thus restricting the size of the opening. Furthermore, an und

e direction, the length of the push rod will be reduced, while the reverse operation will increase the length of the rod or stem. This is primarily intended for taking up any wear that may occur at the ends

cated at the side, after first ascertaining that the end of the short push rod is resting firmly against the periphery of the cam. The play will be apparent, of course, only when the valve is tightly closed, and in order to make certain that t

rating the exhaust valves will be found on one side of the crank case, while that for opening the inlet valves will be located on the other. If the motor is of the "L"-head type, all the cams will be placed on the one

aluminum case. If the cam shaft is driven at the same speed as is the crank shaft of the motor, it will be seen that the valves will open once at every revolution of the flywheel. In a four-cycle motor,

he crank shaft has but one-half as many teeth as have those attached to the cam shafts. There is thus one revolution of each

e crank shaft. This constant relation is necessary in order that the valves will open and close at the proper points during the travel of the piston. For example, the exhaust valve should open tow

ce of the explosion would be wasted almost entirely. On the other hand, the inlet valve should open at about the beginning of the suction stroke in order that the fresh charge

urther adjustment in this respect until after the engine is "taken down" for the purpose of cleaning or the renewal of a broken part. If it should ever become necessary to remove one of the cam shafts or any of the gears constituting the forward tra

without first marking the intermeshing teeth of its driving gear and those of its companions. This may best be done by means of a small prick punch which, when tapped lightly with a hammer, will make a permanent mark at the desired point on the surfa

ft. Three teeth should be marked on each pair of intermeshing gears. That is, a tooth on one gear should be marked, and then ea

l of one from the other. Those cams which are keyed to their shafts are accurately and rigidly set and the keyways so cut that there is s

se marks a pointer attached to the crank case and indicating the top of the flywheel is used. When the line on the flywheel marked, for example, 4 Ex 0, is under the pointer, it indicates t

it is the valve itself that should open after the proper mark on the flywheel has been passed, and that the movement of a long push rod is not sufficient evidence that the valve is beginning to leave its seat. There may be so gre

degrees after the beginning of the suction stroke. This would indicate that the flywheel should be turned through an arc of ten degrees from the point at which the piston is at its upper dead center before the inlet valve for that particular cylinder should begin to open. Expressed in terms o

rtical shaft and two sets of bevel gears. On such motors both inlet and exhaust valves are located in the cylinder heads, and owing to the proximity of the cam shaft, but short p

cams of such a shaft are generally enclosed in oil and dustproof casing screwed to the top of the cylinders. Such a cam shaft sho

h a plug may be removed with a heavy wrench, and as the opening which it fills is larger than the head of the valve, the latter may be removed after first loosening the spiral spring surrounding its stem. It is not necessary to remove the valve entirely from its pocket in order to

at are generally placed in a removable "cage" that either screws in place or is held firmly in position by means of a clamp or like device. Inasmuch as the seat is contained

h a spring, a special tool is required to compress it sufficiently to enable the valve to be removed. A spiral spring that has become weakened may sometimes be strengthened by "stretching," but it is not to be supposed that this would be of great avail in the case of a spring as heavy as those use

ine cylinder. It is not necessary that these guides shall be packed or that they shall be particularly tight, as they are not called upon to retain any gas or air pressure, but they must hold the stem and rod sufficiently rigid to prevent any perceptible side motion and thus cause imperfect seating of the valve. In replacing valve stems and push rods, it should be made certain that each works freely in its guide before the spring is installed. If there i

shaft of the motor. The eccentric replaces the cams of the poppet valve motor, and as it must maintain a certain relation with the position of the piston in order that the operation of the valves shall be timed co