On some designs a double cam is used which serves to operate both the inlet and exhaust valves of the cylinder. The bearings and cams of such a shaft are generally enclosed in oil and dustproof casing screwed to the top of the cylinders. Such a cam shaft should never be dismounted without first marking intermeshing teeth of all spur and bevel gears that are concerned in its operation.

All poppet valves must be accessible and readily removable for the purpose of cleaning and grinding the contact surfaces of the head and seat. The pockets in which the valves placed at the side of a cylinder are located are generally provided with large screw plugs at the top. Such 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the valve entirely from its pocket in order to grind its surfaces, but the pin holding the spring stop in place must be withdrawn so that the tension of the spring on the valve will not be so great as to prevent the latter from being lifted to permit the introduction of the abrasive and turning the head with the grinding tool.

Valves located in the head of the cylinder must be removed entirely before their surfaces can be ground. This, however, is not a difficult operation, as the valve and its seat 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 in this removable cage in which the valve operates, the grinding may be done at a work bench or on the bed of any convenient tool, independently of the location of the motor.

If a valve seems sluggish in its action at high speeds of the motor, it is possible that its spring has become somewhat weakened. These springs are designed to be exceedingly stiff and heavy, some of them requiring a pressure of two hundred and fifty pounds to compress the coils one inch. With such 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 used on some valves. If, however, a flat tool is introduced between the various coils and each is separated slightly so that the ultimate length of the entire spring is greater than it was formerly, it will exert a more powerful force on the valve when it is returned to its place surrounding the stem.

Stiffening the spring, however, will be of but little help if the stem or push rod is tight in the guides through which it slides. These guides are often made of a special bearing bronze and are designed to withstand a large amount of wear, but the friction surfaces must be lubricated if satisfactory service is to be obtained. The lower guide is generally lubricated by the oil from the cams, while the guide near the valve may receive its oil from the engine 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 is a slight tendency for the guide to grip the rod or stem, the latter should be smoothed with emery paper at the point at which it comes in contact with the guide and plenty of oil applied until the surfaces are well "worked down." As the distance that the rods and stems travel through the guides is comparatively short, the wear is slight and only a small amount of lubricant is needed, provided the rubbing surfaces are smooth and well-fitted to each other.

The mechanism of a sleeve valve motor is slightly different from that of the poppet valve type. Each sleeve is operated by a connecting rod and eccentric mounted on a shaft driven by a chain or gears from the crank 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 correctly, the same care must be observed in replacing the eccentric shaft with the proper teeth of the sprocket or gear in mesh as has already been described in connection with the cam shaft of the poppet valve motor.

[CHAPTER III]
Bearings

In the general meaning of the term, a bearing is any part that carries weight or pressure and at the same time rubs over another surface. According to this definition, the portion of the cylinder walls traversed by the pistons are bearings, and that is in reality the case, but the term has come to be applied more specifically to the part of the machine in which another part revolves, either continuously or intermittently. Thus the portions of the crank shaft on which it is supported and the parts of metal in which they revolve combine to form the crank shaft bearings. The shaft or stud on which a gear or wheel is mounted and on which it revolves is the bearing of that gear or wheel.

Although they are concealed, as some six-cylinder motors may be provided with as many as three dozen, or more, bearings—if we consider those on which the cam, pump, and magneto shafts and the gears are mounted— but what descriptions, rules, and precautions apply to all hold true in the largest sense when the crank shaft, connecting rod, and wrist pin bearings only are considered. It is on this latter class that the greatest wear of the motor is concentrated, and the owner who understands and inspects these need fear no trouble from the cam shaft and gear bearings.