Fig. 15—A Piston, Piston Ring, and Piston Pin.
Owing to the fact that both the cylinder walls and piston get hot, and that iron expands and contracts according to its temperature, it is not possible to make a piston alone which would remain air-tight all the time. Engineers, therefore, found it necessary to put rings, which were cut at some point in their circumference, on the outside of the piston itself. These piston rings, due to the fact that they are cut, can accommodate themselves to the varying diameters of the cylinder, and can therefore keep an air-tight fit, even when the piston is moving back and forth all the time. Most of you, no doubt, know that the plunger in a pump is made air-tight by one or a set of leather washers, which, owing to their pliable structure, can expand or contract so as to always fit air-tightly the pipe in the pump. Piston rings work in precisely the same manner, and are always kept lubricated so that they will work smoothly, thus doing away with any friction which might result.
Fig. 16—A section of a Piston, showing location of piston pin and end of connecting rods.
THE CRANK SHAFT
Most of you are familiar with a crank as applied to a grindstone. A crank in a motor is practically the same shape except that it is supported on two bearings instead of one and is therefore made in the form shown in [Fig. 17]. The crank shafts for two and four-cylinder motors are only a combination of two or four of these single cranks. Crank shafts are made up of steel, carefully forged, and then turned and ground down to proper size to fit the bearings for which they are intended. They are hardened and every precaution taken to keep them from wearing. They form one of the most important parts of the motor because they change the back and forth motion of the piston into the rotary motion of the fly wheel. The fly wheel in our former illustration was represented by the grindstone itself. In the real motor the fly wheel is made of cast iron, and after being carefully balanced so that it turns evenly, it is securely bolted to the crank shaft, so that they practically form one piece.
Fig. 17—A Four-cylinder Crank Shaft.
THE CONNECTING ROD
The connecting rod, as you can guess from its name, forms the connecting link between the piston and crank shaft, transferring the energy of the explosive gas, acting behind the piston, to the crank shaft and fly wheel, from which it can be transmitted to the driving wheels of the automobile. It is made up in some such form as shown in [Fig. 18] and is made of steel or bronze. It has a bearing at each end, the smaller one fitting around the piston pin, the larger one surrounding a portion of the crank shaft called the crank pin. Both of these bearings are lubricated by oil which splashes up from the bottom of the crank case when the engine is running. You will notice that one of the bearings is cut in two and bolted together so that you can take it off from the crank shaft, should you wish to examine it.