LO 7-3 Identify and explain the purpose and functions of major piston construction features

LO 7-03: Identify and explain the purpose and functions of major piston construction features.

Piston Construction

A piston can be made using either a casting or a forging process. In a casting process, molten metal is injected into a mold from the bottom up, minimizing the likelihood of trapped air pockets forming in the casting. Various components can be fixed in the piston during the casting process, including reinforcing steel struts in the pin boss area or hardened nickel alloy inserts for ring grooves. When cooled, the piston features are machined to the correct dimensions and to accept rings and wrist pins.

Cast pistons have a limited ability to withstand a sudden rapid rise in cylinder pressure that shock loads the piston in a diesel engine. This explains why most diesel pistons are more commonly made by an alternative process called forging . As the name suggests, forging involves heating and hammering the piston into a final shape. When forged pistons are made from aluminum, aluminum alloy billets are stamped into shape by forging dies. In the forging process, the billet is heated to a plastic-like state, placed into a mold, and then hammered or squeezed under high pressure into a finished shape Figure 16. The forging process realigns the grain structure of the metal and compacts the metal structure to form a denser, heavier piston. Forged pistons are able to withstand higher cylinder pressures and temperatures than cast pistons that have a more crystalline metal grain structure; however, the denser structure of a forged piston will not conduct heat away as fast as a cast piston; a forged piston will run 20% hotter. Forged aluminum pistons also have a greater expansion coefficient than cast pistons and therefore require larger running clearances in cold engines.

FIGURE 07-16: Forged pistons have parting lines where the halves of the stamping dies are separated.

FIGURE 07-17: The finely machined lines on this piston skirt are used to retain oil for lubrication.

A few pistons may have the crown and skirt manufactured in two separate processes. The skirt may be forged and the crown cast, but both will be bonded together by welding or by simply casting the crown onto the forged skirt.

After shaping through forging, the skirts of aluminum pistons are anodized to resist wear. Anodizing is a process used to harden aluminum by electrochemically reacting oxygen with aluminum to form an aluminum oxide outer layer. To get an idea of how tough anodized aluminum skirts are, remember that aluminum oxide is used to make grinding wheels and sandpaper grit. A micro-finish etching, or machining of fine lines, is commonly machined into the piston skirt to retain oil Figure 17. Oil trapped on the skirt is used to help lubricate the piston as it glides over the cylinder wall.

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