We will use for purposes of illustration the common four-cylinder, four cycle, cast en bloc, “L”-head type of motor, as this type is used probably by 90% of the automobile manufacturers. The block of this type of motor is cast with an overlapping shoulder at the upper left hand side which contains a compartment adjoining the combustion chamber in which the intake and exhaust valves seat, and the casting is made, in the shape of the Capital letter L turned upside down. This arrangement allows both valves to seat in one chamber and to operate from one cam shaft.

The operation of each cylinder is identically the same whether you have a one or a many cylindered motor, consequently when you have gained a working knowledge of one cylinder, others are a mere addition. This may sound confusing when the eight or twelve cylindered motor is mentioned, but is more readily understood when we consider the fact that an eight or twelve cylindered motor is nothing more than two fours or two sixes, set to a single crank-case or base in V-shape to allow the connecting rods of each motor to operate on a single crank shaft. This arrangement also allows all the valves to operate from a single cam shaft, thereby making the motor very rigid and compact, which is an absolute necessity considering the small space that is allowed for the motor in our present-day designs.

Fig. 1. The casting or block, which is the foundation of the whole motor or engine, usually has a removable head which allows for easy access to the pistons and valves. The block is cast with a passage or compartment through the head and around the cylinders through which water circulates for cooling the adjoining surfaces of the cylinders. This alleviates the danger from expansion and contraction caused by the tremendous heat generated in and about the combustion chambers. This block also contains the cylinders and valve seats. The pistons and valves are fitted to their respective positions as construction progresses.

Fig. 2. The block with head removed shows the smooth flush surface of the block face and the location of the cylinders in which the pistons operate or slide, with each power impulse or explosion. When the piston is at its upper extreme it comes within a sixteenth of an inch of being flush with the top of the block, while the valves (also shown in Fig. 2) rest on ground-in seats, in their respective chambers, and are operated by a stem which extends downward from the head through a guide bushing in the block to the cam shaft.

The location of the water vents is also shown, through which water is circulated to prevent the cylinders from overheating which would cause the pistons to “stick” from expansion.

Fig. 3. The top or head of the motor is removed, exposing the combustion chambers. These chambers must be absolutely air-tight as the charge of gas drawn in through the inlet valve is compressed here before the explosion takes place, and low compression means a weak explosion, which causes the motor to run with an uneven-jumpy motion, and with an apparent great loss of power. A copper fiber insert gasket is placed between the top of the block and the head before it is bolted down. This gasket prevents any of the compression from escaping through unevenness of the contact surfaces, as metal surfaces are prone to warp when exposed to intense heat. It is necessary to turn the bolts in the head down occasionally, as the heat causes expansion. The following contraction, which loosens them, results in a loss of compression and a faulty operation of the motor.

The spark-plug vents through the head are usually located directly over the piston although in some cases they are over the valve head and in some motors which are cast without a removable head they may be at one side of the combustion chamber. The location of the spark-plug does not materially affect the force of the explosion, although when it is located directly over the piston a longer plug may be used, as the pistons do not come up flush with the top of the block, and a spark-plug extended well into the combustion chamber will not become corroded with carbon or burnt oil as is usually the case with a plug which does not extend beyond the upper wall surface of the combustion chamber.

Fig. 4. The plunger or piston is turned down to fit snugly within the cylinder and is cast hollow, with two shoulders extending from the inside wall.

Fig. 4A shows a split piston. Three grooves are cut into it near the head to receive the piston rings. The width and depth of these grooves vary according to the size of the piston. A hole is bored through the piston and shoulders about half way from each end. The bushing or plain bearing shown in Fig. 4B is pressed into this hole and forms a bearing for the wrist pin also shown in Fig. 4B. Wrist pins are usually made of a much softer metal than the bearing, and are subjected to severe duty, which often causes them to wear and produce a sharp knock; this may be remedied by pressing out the pin, giving it a quarter turn, and replacing it in that position.

Fig. 5 shows a split joint piston ring. Piston rings are usually made from a high grade gray iron, which fits into the grooves in the piston and springs out against the cylinder walls, thereby preventing the compressed charge of gas from escaping down the cylinder, between the wall and the piston. Fig. 5A shows a piston equipped with leak-proof rings; this type of piston ring has overlapping joints, and gives excellent service, especially when used on a motor which has seen considerable service. Fig. 5B illustrates how piston rings may line up, or become worn from long use, or from faulty lubrication. This trouble may be easily detected by turning the motor over slowly. The escaping charge can usually be heard and the strength required to turn the motor will be found much less uniform on the defective cylinder.

The motor should be overhauled at least once every year, and by applying new rings to the pistons at this time new life and snappiness may be perceived at once.

The connecting rod shown in Fig. 6 has a detachable or split bearing on the large end, and takes its bearing on the crank pin of the crank shaft. The small or upper end may have either a hinge joint or press fit to the wrist pin. This rod serves as a connection and delivers the power stroke from the piston to the crank shaft. These rods are required to stand very hard jars caused by the explosion taking place over the piston head. The bearings are provided with shims between the upper and lower half for adjusting. Piston or connecting rod bearings must be kept perfectly adjusted to prevent the bearings from cracking or splitting which will cause the rod to break and which may cause considerable damage to the crank case.

Fig. 7 shows a counter balanced crank shaft. This type of crank-shaft is provided with weights which balance the shaft and carry the momentum gathered in the revolution.

Fig. 8 shows the plain type of crank shaft with the timing gear attached to the front end and the fly-wheel attached to the rear end. The crank shaft shown is carried or held by five main bearings, which is an exception, as the majority of motor manufacturers use only three main bearings to support the crank shaft, while in some of the smaller motors only two are used. These bearings are always of the split type, the seat for the upper half is cast into the upper part of the crank-case, and the lower half is usually attached to the upper half by four bolts which pass through the flange at each side of the bearing. Small shims of different sizes are employed between the flanges of each half of the bearing in order to secure a perfect adjustment which is very essential, as these bearings are subjected to heavy strains and severe duty. A shim may be removed occasionally as the bearing begins to show wear. A worn main bearing can be detected by placing the metal end of a screw-driver or hammer on the crank-case opposite the bearing and the other end to the ear. If the bearing is loose or worn a dull bump or thud will be heard. This looseness should be taken up by removing a shim of the proper thickness.

Main bearings run loose for any length of time will be found very hard to adjust as the jar which they are subjected to invariably pounds them off center which makes readjustment a very difficult task to accomplish with lasting effect. New main bearings in a motor should always be scraped to secure a perfect fit. A loose piston or connecting rod bearing will produce a sharp knock which can easily be determined from the dull thud produced by a loose main bearing. (Fig. 9.) The cam shaft revolves on bearings and is usually located at the base of the cylinders on the left hand side looking toward the radiator and carries a set of cams for each cylinder. The cam pushes the valve open, and holds it in this position, while the piston travels the required number of degrees of the cycle or stroke.

The cam shaft is driven from the crank shaft usually through a set of timing gears, and operated at one-half the speed of the crank shaft in a four cycle motor, as a valve is only lifted once, while the crank shaft makes two revolutions or four strokes. The cam-shaft bearings, and the timing gears are usually self-lubricating and require very little attention. Timing of the cam shaft is a rather difficult matter and will be treated in a following chapter under the head of valve timing.

The oil pan or reservoir forms the lower half or base of the crank case. The lubricating oil is carried here at a level which will allow the piston rods to dip into it at each revolution of the crank shaft. The timing gears receive their lubrication from the supply carried in the reservoir by means of a plunger or piston pump which is operated from the cam shaft. The balance of the motor is usually lubricated by a splash system taken up in a later chapter on lubrication. The oil is carried at a level between two points marked, high and low, on a glass or float gauge which is located on the crank case. A gasket made of paper or fiber is used between the union or connection of the oil reservoir and the upper half of the crank case to prevent the oil from working out through the connection.

Fig. 10 represents the flywheel. The flywheel is usually keyed to the crank shaft directly behind the rear main bearing. This wheel is proportionate in weight to the revolving speed of the motor, which it keeps in balance by gathering the force of the power stroke. The momentum gathered by it in this stroke carries the pistons through the three succeeding strokes called the exhaust, intake, and compression strokes. The flywheel also serves as a connection between the power-plant and the running gear of the car, as a part of the clutch is located on it, and the connection takes place either in the rim or on the flange.