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and seizing of the cylinders and pistons, and this is accomplished either by means of air forced past the cylinders by a fan or blower, or by water circulated between the cylinder proper and an outer covering known as the water jacket (Fig. 38 B).
Air-cooled motors are usually provided with flanges of thin metal cast on the cylinders (Fig. 39), to aid in radiating the heat, and are little used in comparison with water-cooled motors. In marine use the watercooling system is used invariably, as it is very easy to connect the pump to an outlet and inlet and thus keep fresh, cool water circulating through the jacket while the motor is operating. When used for vehicle or stationary use a radiator or similar cooling system is used, through which the water is forced by the pump, and which, by numerous thin flanges or by thin-walled compartments of small capacity, tends to radiate heat and thus cool the water. In stationary engines a large tank, or hopper, is usually sufficient to cool the water through exposure of a large surface to the air. A sheet of cloth, wire gauze, or a perforated plate is often used to aid in cooling the water from stationary motors, as the hot water from the engine flowing over this is rapidly cooled by radiation of heat from the large surface exposed to the air.
Ignition of either the jump spark or make-and-break spark may be used in gasolene motors and as far as efficiency is concerned there is but little choice; the make-and-break system is electrically simple and mechanically complicated, while the jump-spark system is just the reverse-electrically complicated and
mechanically simple. Ignition systems will, however, be dealt with in detail in a following chapter and are only mentioned here in connection with the various parts of the motors.
The actual value, life, and power of a motor depend almost as much upon the quality of material used and care taken in the construction of its various parts as upon its design and proportions. The cylinders are commonly made of fine-grained cast iron, and after boring to the proper size should be ground to a mirror finish and fitted to within 1/1000 of an inch. The piston may vary considerably in shape and proportion with different makes and types of engines, but the general principle of construction is the same, and cast iron is principally used in making them. A perfect-fitting piston is essential to a good motor, for if too loose the compression will be lost, whereas, if too tight, it will bind when hot and score the cylinder walls or prevent the motor from operating. It is customary to test all pistons Fig. 41.-Piston and Piston
Rings and cylinders by limit gauges; one of these gauges is 1/2000 of an inch oversize, the other the same amount undersize, and if the parts fit the undersize gauge or fail to fit the oversize gauge they should be
discarded. The piston should not fit the cylinder too snugly, however, for allowance must be made for expansion when hot, and in order to allow for this and yet to retain gas under compression, piston rings are provided. These rings are made of cast iron and are turned and faced on a lathe, ground on the sides, and are then cut, clamped together, and ground on the faces. They are made eccentric,-thicker on one side than the other,and are cut with a diagonal or lapped joint on the thinner
side. The rings may be two, three, or four in number and are placed in grooves on the piston and are usually pinned in position (Fig. 41). The rings are slightly compressed when the piston is inside of the cylinder and their tendency to expand keeps them pressed firmly against the cylinder walls, thus forming a gas-tight joint.
The connecting-rod may be either of steel or bronze, and may be either cast or forged. The upper end, which fits inside the piston, is held in place by a hardened pin passing through the piston from one side to the other and known as the piston pin (Fig. 42). There are various methods of fastening this pin in place, but set-screws within the piston (Fig. 43, S) are perhaps the most satisfactory. Some makers bush the pin where it fits the piston walls and fasten the connecting-rod to the pin (Fig. 44), while others fasten the pin to the piston and provide a bearing surface for the connecting-rod
head by placing a bronze bushing between the pin and connecting-rod (Fig. 45). The latter system is preferable as it obviates any danger of the pin working endwise and scoring the cylinder walls. The lower end of the connecting-rod is split, or cut, through the centre of the hole bored for the crank shaft, and a babbitt, or bronze, bearing fitted in place and the two parts clamped together around the crankshaft. There are various methods of fastening the bottom cap to the rod proper,
Fig. 43.- Piston-pin held by Set-screws
but a hinge on one side and cap-screw with double nuts on the other is widely used (Fig. 46). Other makers used a loose cap held to the rod by screws on either side, and while this method allows of finer adjustment, it is not so convenient as the hinged cap (Fig. 47). Crank cases may be cast from iron, steel, aluminum or other metal, and may be made either solid, with one end removable (Fig. 48); with split base (Fig. 49); with both
bearings or end plates separate (Fig. 50); or the upper portion of crank case and cylinder may be cast in one piece with a separate cylinder head held in place by bolts (Fig. 51); or a combination of two or more of the above
Fig. 44.—Piston-pin bushed in
Fig. 45.- Piston-pin bushed
may be used. In either case all joints should be turned and faced true and smooth and thin gaskets of paper, or similar packing, placed between the ground surfaces.
Crank cases are commonly furnished with hand-hole plates on the sides (Figs. 50, 51, H), and these should be of ample size to permit tightening or adjusting the