The moving parts of an engine serve an important function in turning heat energy into mechanical energy. They further convert reciprocal motion into rotary motion. The principal moving parts are the piston assembly, connecting rods, crankshaft assembly (includes flywheel and vibration dampener), camshaft, valves, and gear train.

The burning of the fuel-air mixture within the cylinder exerts a pressure on the piston, thus pushing it down in the cylinder. The action of the connecting rod and crankshaft converts this downward motion to a rotary motion.

Figure 12-13.Piston and connecting rod (exploded view).

a carrier for the piston rings used to seal the compression in the cylinder. (See. fig. 12-13.)

The piston must come to a complete stop at the end of each stroke before reversing its course in the cylinder. To withstand this rugged treatment and wear, it must be made of tough material, yet be light in weight. To overcome inertia and momentum at high speed, it must be carefully balanced and weighed. All the pistons used in any one engine must be of similar weight to avoid excessive vibration. Ribs are used on the underside of the piston to reinforce the hand. The ribs also help to conduct heat from the head of the piston to the piston rings and out through the cylinder walls.

The structural components of the piston are the head, skirt, ring grooves, and land (fig. 12-14). However, all pistons do not look like the typical one illustrated here. Some have differently shaped heads. Diesel engine pistons usually have more ring grooves and rings than gasoline engine pistons. Some of these rings may be installed below as well as above the wrist or piston pin (fig. 12-15).

Fitting pistons properly is important. Because metal expands when heated and space must be provided for lubricants between the pistons and the cylinder walls, the pistons are fitted to the engine with a specified clearance. This clearance depends upon the size or diameter of the piston and the material form which it is made. Cast iron does not expand as fast or as much as aluminum. Aluminum pistons require more clearance to prevent binding or seizing when the engine gets hot. The skirt of bottom part of the piston runs much cooler than the top; therefore, it does not require as much clearance as the head.

The piston is kept in alignment by the skirt, which is usually cam ground (elliptical in cross section) (fig.12-16). This elliptical shape permits the piston to fit the cylinder, regardless of whether the piston is cold or at operating temperature. The narrowest diameter of the piston is at the piston pin bosses, where the piston skirt is thickest. At the widest diameter of the piston, the piston skirt is thinnest. The piston is fitted to close limits at its widest diameter so that the piston noise (slap) is prevented during the engine warm-up. As the piston is

expanded by the heat generated during operation, it becomes round because the expansion is proportional to the temperature of the metal. The walls of the skirt are cut away as much as possible to reduce weight and to prevent excessive expansion during engine operation. Many aluminum pistons are made with split skirts so that when the pistons expand, the skirt diameter will not increase.

The two types of piston skirts found in most engines are the full trunk and the slipper. The full-trunk-type skirt, more widely used, has a full cylindrical shape with bearing surfaces parallel to those of the cylinder, giving more strength and better control of the oil film. The slipper-type (cutaway) skirt has considerable relief on the sides of the skirt, leaving less area for possible contact with the cylinder walls and thereby reducing friction.

Three methods are commonly used for fastening a piston pin to the piston and the connecting rod: fixed pin, semifloating pin, and full-floating pin (fig. 12-17). The anchored, or fixed, pin attaches to the piston by a screw running through one of the bosses; the connecting rod oscillates on the pin. The semifloating pin is anchored to the connecting rod and turns in the piston pin bosses. The full-floating pin is free to rotate in the connecting rod and in the bosses, while plugs or snap-ring locks prevent it from working out against the sides of the cylinder.

The principal function of a compression ring is to prevent gases from leaking by the piston during the compression and power strokes. All piston rings are split to permit assembly to the piston and to allow for expansion. When the ring is in place, the ends of the split joint do not form a perfect seal; therefore, more than one ring must be used, and the joints must be staggered around the piston. If cylinders are worn, expanders (figs. 12-15 and 12-18) are sometimes used to ensure a perfect seal.

The bottom ring, usually located just above the piston pin, is an oil-regulating ring. This ring scrapes the excess oil from the cylinder walls and returns some of it, through slots, to the piston ring grooves. The ring groove under an oil ring has openings through which the oil flows back into the crankcase. In some engines, additional oil rings are used in the piston skirt below the piston pin.