Figure 12-13.—Piston and connecting rod (explodedview).a carrier for the piston rings used to seal thecompression in the cylinder. (See. fig. 12-13.)The piston must come to a complete stop at theend of each stroke before reversing its course in thecylinder. To withstand this rugged treatment andwear, it must be made of tough material, yet be lightin weight. To overcome inertia and momentum athigh speed, it must be carefully balanced andweighed. All the pistons used in any one engine mustbe of similar weight to avoid excessive vibration. Ribsare used on the underside of the piston to reinforcethe hand. The ribs also help to conduct heat from thehead of the piston to the piston rings and out throughthe cylinder walls.The structural components of the piston are thehead, skirt, ring grooves, and land (fig. 12-14).However, all pistons do not look like the typical oneillustrated here. Some have differently shaped heads.Diesel engine pistons usually have more ring groovesand rings than gasoline engine pistons. Some of theserings may be installed below as well as above thewrist or piston pin (fig. 12-15).Fitting pistons properly is important. Becausemetal expands when heated and space must beprovided for lubricants between the pistons and thecylinder walls, the pistons are fitted to the enginewith a specified clearance. This clearance dependsupon the size or diameter of the piston and thematerial form which it is made. Cast iron does notexpand as fast or as much as aluminum. Aluminumpistons require more clearance to prevent binding orseizing when the engine gets hot. The skirt of bottompart of the piston runs much cooler than the top;therefore, it does not require as much clearance asthe head.Figure 12-14.—The parts of a piston.12-14
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business