The foundation of an engine is the base, which supports the bearings in which the crank shaft revolves, and to which the cylinders are attached. The cylinders of tractor engines are made of cast-iron, and the cylinder heads, which close the upper ends of the cylinders, are usually in a separate piece, bolted on. The joint between the cylinders and the cylinder head is made tight by placing between them a gasket of asbestos and thin sheet metal.

The crank shaft has as many cranks, or throws, as the engine has cylinders. Crank shafts for 2-cylinder engines are shown in Figure 7; the upper one is for an engine of the type shown in Figure 3, with pistons moving in the same direction. With both cranks projecting from one side the shaft is out of balance, so balance weights are attached to the opposite side.

The other shaft shown in Figure 7 does not need balance weights, for one crank balances the other. A four-cylinder crank shaft, Figure 8, is also in balance.

Crank shafts revolve in main bearings, which are set in the engine base. In tractor engines these are usually plain bearings, a half of such a bearing being shown in Figure 9. This is a bronze shell lined with a softer metal, making an exact fit on the shaft; with the two halves in place, the shaft should turn freely, but without looseness or side play. The grooves shown are to admit lubricating oil.

The piston is attached to the crank shaft by a connecting rod, which is illustrated in Figure 10. Pistons are shown in Figures 11 and 12; they are made as light as is consistent with the pressure that they must bear, and are hollow, and open at the lower end.

The piston is attached to the connecting rod by a wrist pin, or piston pin, which is a shaft passing through it from side to side, and also through the bearing in the upper end of the connecting rod. The connecting rod swings on the wrist pin in following the rotation of the crank shaft, and its attachment to the wrist pin must permit this without being loose.

The bearings at the two ends of a connecting rod are usually adjustable, so that wear can be taken up; some of the methods of doing this are illustrated in Figure 10. In A, the wrist pin bearing is a plain tube, ground to an exact fit; when it is worn it must be replaced. In B, the bearing is split, and the ends are drawn together by a bolt to the correct fit. The bearing in C is in two parts, held together by a U-shaped bolt, while in D the two parts are held together by a cap bolted to the end of the connecting rod. In E, the end of the connecting rod is a square loop enclosing the two parts of the bearing; the parts are held in the proper position by a wedge adjusted by screws.

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.

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.

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.

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.

The valve in Figure 13 is held on its seat by a spring. The cam bears against the end of the valve stem, and as it revolves its bulge forces the valve stem and valve to move endways and thus to uncover the valve opening.

As the movement of the piston depends on the crank shaft, the valve can be made to open at the right time by a proper setting of the gears that drive the cam shaft.

The length of time that the cam will hold the valve open depends on the shape of the bulge of the cam. It can be seen that the pointed cam of Figure 13 will not hold the valve open for as long a time as the flat-end cam of Figure 14.

In the design shown in Figure 13 the cam bears directly against the end of the valve stem, the cam shaft in this case lying along the cylinder head. In the construction shown in Figure 15 the valves are not placed in the cylinder head, but are in an extension or valve pocket projecting from the combustion chamber; this cam shaft is near the crank shaft. It would not be practicable to make the valve stem long enough to reach down to the cam, so a length of rod, called a push rod, or tappet, is placed between them; the cam moves the push rod and the push rod in turn moves the valve. This is a construction frequently used for automobile engines.

In tractor engines the cam shaft is usually placed near the crank shaft, as in Figure 15, and the valves are in the head, so that a valve moves in the opposite direction to the movement of the push rod. This requires still another part to be used, called the rocker arm. It is shown in Figure 16. It is a short bar, pivoted at or near the center, with one end at the push rod and the other at the valve stem. When it is moved by the push rod it in turn moves the valve.

Valves operated by push rods and rocker arms are also shown in Figures 17, 18 and 19; Figure 18 is a single-cylinder horizontal engine, while Figure 19 is a horizontal double opposed engine, in which one cam operates a valve in each cylinder. Figure 20 shows the valve mechanism of a vertical engine in which all parts, including the rocker arm, are enclosed to protect them from dust, and so they can run in oil.

A small space is always left somewhere between the cam and the valve stem, to give the valve stem room to lengthen, which it will do when it gets hot. If this space were not left, the valve stem, in lengthening as it became hot, would strike the part next to it, and the valve would be lifted from its seat. This would cause the engine to lose power. This space must be kept properly adjusted, and instructions for this will be found in Chapter XII.

A valve is held against its seat by a spring, which must be compressed when the valve is opened. If this spring is too weak, it will not hold the valve tightly on its seat, while if it is too stiff, the cam shaft and other parts will be needlessly strained in compressing it.

Friction between the cam and the end of the valve stem or push rod would cause rapid wear if these parts were not of hardened steel, and kept well oiled. Still further to reduce wear, there is usually a roller on the end of the push rod, as shown in Figure 16 and some of the other illustrations. Figure 15 shows a construction in which the end of the push rod is a flat disk, which rotates as the cam comes into contact with it.

When the mixture burns, the top of the piston, the cylinder head, and the walls of the combustion chamber become heated, and if it is not prevented they will get so hot that they will expand sufficiently to cause the piston to stick, or seize. The upper part of the cylinder is, therefore, provided with a cooling system that keeps these parts from getting overheated. Channels are provided through which water is circulated; the water takes the heat from the metal parts, becoming heated itself, and then passes to a cooler, or radiator, where it gives up the heat to currents of air.

In addition to the channels, or water jackets, around the cylinder, a cooling system includes the radiator, the connections, and usually a pump that keeps the water in motion.

In some tractors, notably the Fordson, no pump is used; the water circulates because it is heated. This is called a thermo-syphon system. When the engine runs, the water in the cylinder jackets becomes heated; as hot water is lighter than cold water, it rises and flows out of the jackets to the radiator, its place being taken by cool water from the bottom of the radiator. This circulation continues as long as the water in one part of the system is hotter than the water in some other part of the system.

The lubrication of an engine is described and explained in Chapter X.