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.
Fig. 7.—2-Cylinder Crank Shaft
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.
Fig. 8.—4-Cylinder Crank Shaft
Fig. 9.—Half of a Plain Bearing
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.