Fig. 6.—Connecting Rod.
The piston is a trifle smaller than the bore of the cylinder, and its length is usually greater than its diameter. It is hollow, with one end closed, the closed end being that against which the pressure is exerted. A wrist pin passes through it, and through the small end of the connecting rod, to enable the latter to swing from side to side in following the turning of the crank shaft. A tight joint is maintained between it and the cylinder walls by cast-iron piston rings, which are of square or rectangular cross-section, split so that they may spring open, and fitted into grooves cut around the piston. They are of such shape that their tendency to expand keeps them pressed against the cylinder walls, but being split, their elasticity prevents their binding; they fit the grooves snugly, and while they may move freely in them, they hold the pressure from escaping. The number of rings varies with the design of the engine, but the most usual arrangement is three to a piston, placed around the upper end.
Fig. 7.—A, Piston; B, piston in section.
Fig. 8.—Piston Rings.
The cylinder should be of the highest grade of cast iron, with the smoothest possible surface for the piston to slide against.
Fig. 9.—Conical Valve Seat.
