The crank shaft bearing of the connecting rod shown in F is in two parts which are hinged together. G, H, and K show the forms usually used in tractor engines, which consist of two parts bolted together. [Illustration: FIG. 12.—WRIST PIN FASTENINGS] The wrist pin is usually firmly attached to the piston, so that the connecting rod swings on it; methods of securing the wrist pin are shown in Figure 12, the wrist pin being held in supports cast in the piston. In A, the wrist pin is held by two set screws, and in B, by pins passing through it. The wrist pin shown in D is hollow, as is very common, and a bolt passes through part of the piston and into the wrist pin. In the construction shown in C the wrist pin is secured to the connecting rod and moves in bearings in the piston. In E, a ring fitting in a groove around the piston prevents the wrist pin from moving endways. The engine must usually be taken to pieces in order to get at the wrist pin; lock nuts, lock washers or cotter pins are always used to prevent the trouble that would be caused if the wrist pin worked loose. A leak-proof joint between the piston and the cylinder is made by means of _piston rings_ that fit in grooves around the piston, as shown in E, Figure 12. Piston ring grooves are shown in Figure 11. Piston rings are not solid, but are split so that they are elastic; they fit snugly in their grooves, and tend to spring open to a greater size than the cylinder. This causes them to maintain a close fit against the cylinder, and the gases are prevented from leaking past. [Illustration: FIG. 13.—VALVE] Each cylinder is provided with two valves: the _inlet valve_ that admits fresh mixture and the _exhaust valve_ through which the burned gases escape. These valves are metal disks with funnel-shaped edges fitting into funnel holes. A valve and its stem are shown in Figure 13 and also in Figure 15. [Illustration: FIG. 14.—ACTION OF A CAM] A valve is opened at the proper time by a _cam_, and closed by a spring. A cam is a wheel with a bulge on one side, so that its rim is eccentric to its shaft, as illustrated in Figure 14, which shows a cam in three positions of a revolution. A rod resting on the rim of the cam is moved endways as the bulge passes under it, and the valve is operated by connecting it with the rod. A valve is opened once during two revolutions of the crank shaft; therefore the cam cannot be placed on the crank shaft, for, if it were, the valve would be opened every revolution. The cam is placed on a separate shaft which is driven by the crank shaft at half its speed. This is usually done with gears, a gear on the crank shaft meshing with a gear on the cam shaft having twice as many teeth; the crank shaft gear must make two revolutions in turning the cam shaft gear once.