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. [Illustration: 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. [Illustration: FIG. 8.—4-CYLINDER CRANK SHAFT] [Illustration: 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. [Illustration: FIG. 10.—CONNECTING ROD BEARINGS] [Illustration: FIG. 11.—PISTON COMPLETE AND IN SECTION] 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.