THE PISTON
The piston forms, as you will recall, the bullet in the cannon, which instead of leaving the barrel, was made to travel back and forth inside of the cylinder under the action of the explosive gas. Owing to the fact that a solid piece of iron would be very heavy and would get very warm, the real piston used in a motor is made hollow so that it is merely a shell. Instead of fastening the rod to the end of it, a small rod, called the piston pin is in the center of it, and to this the connecting rod is connected. [Fig. 16] shows a section of the piston. You will notice that the piston pin is kept from sliding sideways by a bolt that is screwed into it.
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.