Piston Assembly

Pistons (fig. 3-22) are usually made of an aluminum alloy. They are a sliding fit in the cylinders. This serves several purposes as follows:

• Transmits the force of combustion to the crankshaft through the connecting rod.
• Acts as a guide for the upper end of the connecting rod.
• Serves as a carrier for the piston rings that are used to seal the compression in the cylinder.

The piston must withstand incredible punishment under temperature extremes. The following are examples of conditions that a piston must withstand at normal highway speed:

• As the piston moves from the top of the cylinder to the bottom (or vice versa), it accelerates from a stop to a speed approximately 50 mph at midpoint, and then decelerates to a stop again. It does this approximately 80 times per second.
• The piston is subjected to pressures on its head in excess of 1,000 psi.
• The piston head is subjected to temperatures well above 600°F.

 The structural components of the pistons are the head, skirt, ring grooves, and lands (fig.3-23); however, all pistons do not look like the typical one shown here. Some have differently shaped heads.Diesel engine pistons usually have more ring grooves and rings than the pistons of a gasoline engine. Some of these rings may be installed below as well as above the wrist or piston pin (fig.3-24).

 Fitting pistons into the cylinder properly is very important. Because metal expands when heated, space must be provided for lubricants between the pistons and the cylinder walls. Pistons must have features built into them to control expansion. Without these features, pistons would fit loosely in the cylinders when cold, and then bind in the cylinders, as they are warmed up. This is the problem with aluminum because it expands so much. The pistons (fig. 3-25) may be designed with the following features to control expansion:

• It is obvious that the crown of the piston gets hotter than the rest of the piston. To prevent it from expanding to a larger size than the rest of the piston, it is machined to a diameter that is approximately 0.03 to 0.04 of an inch smaller than the skirt area.
• One way to control expansion in the skirt area is to cut a slot up the side of the skirt. As a split-skirt piston warms up, the split merely closes, thereby keeping the skirt from expanding outward and binding the piston in the cylinder.
• Another variation of the split-skirt piston is the T-slot piston. The T-slot piston is similar to the split-skirt piston with the addition of a horizontal slot that retards heat transfer from the piston head to the piston skirt.
• Some aluminum pistons have steel braces cast into them to control expansion.
• The skirt, or 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), as shown in figures 3-26 and 3-27. By making the piston egg-shaped, it is able to fit the cylinder better throughout its operational temperature range. Cam-ground pistons are machined so their diameter is smaller and more parallel to the piston pin axis than it is perpendicular to it. When the piston is cold, it is big enough across the larger diameter to keep from rocking. As it warms up, it expands across its smaller diameter at a much higher rate than at its larger diameter. This tends to make the piston round at operating temperature. The walls of the skirt are cut away as much as possible to reduce weight and to prevent excessive expansion during engine operation. Virtually all pistons in automotive applications are cam ground.

There are two types of piston skirts in most engines—full trunk and partial skirted (slipper) (fig. 3-28). The full trunk type of skirt has a full cylindrical shape with hearing surfaces parallel to those of the cylinder. This gives it more strength and better control of the oil film. The partial skirt or slipper skirt has considerable relief on the sides of the skirt. Removal of the skirt in these areas serves the following purposes:

• Lightens the piston, which, in turn, increases the speed range of the engine.
• Reduces the contact area with the cylinder wall, which reduces friction.
• Allows the piston to be brought down closer to the crankshaft without interference with its counterweights.

The piston pin (fig. 3-29) serves to connect the piston to the connecting rod. It passes through the pin bosses in the piston and the upper end of the connecting rod. The piston pin must be hard to provide the desired wearing qualities. At the same time, the piston pin must not be too brittle. A case-hardened steel pin is the best to satisfy the overall requirements of a piston pin. Case hardening is a process that hardens the surface of the steel to any desired depth. The pin is also hollow to reduce the overall weight of the reciprocating mass. They are lubricated by splash from the crankcase or by pressure through passages bored in the connecting rod.

There are three methods used for fastening a piston to the connecting rod. The following are the three different types of piston pins (fig. 3-30):

1. An anchored, or fixed, piston pin is locked into the piston pin bosses by a screw. The rod pivots freely on the connecting rod, which is fitted with a bronze bushing.
2. A semifloating pin is locked to the connecting rod by a screw or by friction. The pin pivots freely in the piston pin bosses.
3. The full-floating piston pin pivots freely in the connecting rod and piston pin bosses. The outer ends of the piston pins are fitted with lock rings to keep the pin from sliding out and contacting the cylinder walls.

Piston rings serve three important functions (fig.3-31). They provide a seal between the piston and the cylinder wall to keep the force of the exploding gases from leaking into the crankcase from the combustion chamber. Blow-by is detrimental to engine performance because the force of the exploding gases merely bypasses the piston, rather than push down on it. It also contains the lubricating oil. They keep the lubricating oil from passing the piston and getting into the combustion chamber from the crankcase. Also, they provide a solid bridge to conduct heat from the piston to the cylinder wall. About one third of the heat absorbed by the piston passes to the cylinder wall through the piston rings.

Piston rings are secured to the piston by fitting into grooves. They are split to allow for installation and expansion, and they exert pressure on the cylinder walls when installed. They fit into grooves that are cut into the piston and are allowed to float freely in these grooves. A piston ring that is formed properly, working in a cylinder that is within limits for roundness and size, exerts an even pressure and a solid contact with the cylinder wall around the entire circumference. There are two basic classifications of piston rings. The compression ring (fig. 3-32) that seals the force of the exploding mixture into the combustion chamber and the oil control ring (fig. 3-32) that keeps engine lubricating oil from getting into the combustion chamber. These rings are arranged on the piston in three basic configurations (fig. 3-33). They are as follows:

The three-ring piston has two compression rings from the top, followed by one oil control ring—the most common configuration.

• The four-ring piston has three compression rings from the top, followed by two oil control rings. Commonly used on diesel engines because they are more prone to blow-by. This is due to the much higher pressures generated during the power stroke.
• The four-ring piston has two compression rings from the top, followed by one oil control ring. The bottom oil control ring may be located above or below the piston pin. This is not very common in current engine design.

There is an additional groove cut into the piston just above the top ring groove. The purpose of this groove is to divert some of the intense heat that is absorbed by the piston head away from the top ring. This groove is called a heat dam.

ring gap (fig. 3-34) is the split in the piston ring. This is necessary for installing the ring on the piston and allowing for expansion from heating. The gap must be such that there is enough space so the ends do not come together, as the ring heats up. This would cause the rings to break. There are a few variations of ring gap joints (fig. 3-35). Two-cycle engines usually have pins in their ring grooves to keep the gap from turning. This is important because the ring would break if the ends were allowed to snap into the inlet or exhaust ports. Staggering the ring gap is also important as it prevents blow-by. A significant amount of total blow-by at the top ring will be from the ring gap. For this reason, the top and second compression rings are assembled to the piston with their gaps 60-degrees offset with the first ring gaps.

Rings must also be fitted for the proper side clearance (fig. 3-36). This clearance varies in different types and makes of engines; however, in a diesel engine, the rings must be given greater clearance than in a gasoline engine. If too much side clearance is given the rings, excessive wear on the lands will result. If there is too little side clearance, expansion may cause the lands to break.

When piston rings are new, a period of running is necessary to wear the piston rings a small amount, so they conform perfectly to the cylinder walls. The cylinder walls are surfaced with a tool called a hone, which leaves fine scratches in the cylinder walls (fig.3-37). The piston rings are made with grooves in their faces, which rub against the roughened cylinder walls, serving to accelerate ring wear during the initial stages.As the surfaces wear smooth, the rings wear in.

Extreme pressure may be applied to high spots on the piston rings during the wear-in period. This can cause the piston rings to overheat at these points and cause damage to the cylinder walls in the form of rough streaks. This condition is called scuffing. New piston rings are coated with a porous material, such as graphite, phosphate, or molybdenum. These materials absorb oil and serve to minimize scuffing. As the rings wear in, the coatings wear off.

Some piston rings are chrome-plated. Chrome-plated rings provide better overall wearing qualities. They also are finished to a greater degree of accuracy, which lets the piston rings wear in faster.

Figure 3-22.—Piston.

Figure 3-23.—The parts of a piston.

Figure 3-24.—Diesel piston assembly.

Figure 3-25.—Controlling piston expansion.

Figure 3-26.—Cam-ground piston action.

Figure 3-27.—Cam-ground piston.

Figure 3-28.—Full- and partial-skirted pistons.

Figure 3-29.—Piston pin.

Figure 3-30.—Types of piston pins.

Figure 3-31.—Purpose of piston rings.

Figure 3-32.—Types of piston rings.

Figure 3-33.—Configurations of piston rings.

Figure 3-34.—Ring gap.

Figure 3-35.—Ring gap variations.

Figure 3-36.—Fitting piston ring and installing piston.

Figure 3-37.—Piston ring wear-in.