Pistons

Figure 3-22.–Checking ring gap clearance. been installed, check the ring-to-land clearance. (See fig. 3-23.) If the clearance is too small, the ring may bind or seize, allowing improper sealing and blowby to occur. If the clearance is excessive, the ring may flutter and break itself or the piston land. After you have properly installed all the rings, coat the entire assembly with oil, then insert it into the cylinder bore. Position the rings so the gap of each successive ring is on an alternate side and the gaps are in line with the piston pin bosses. On large engines, use a chain fall to hold the piston assembly in position as you lower it into the cylinder. (See fig. 3-24.) Figure 3-23.–Checking ring groove side clearance. 75.56 Figure 3-24.–Installing a piston in a cylinder bore with a funnel-type ring compressor. When a piston is being inserted into a cylinder, the piston rings must be compressed evenly. Special funnel-type tools, similar to the one shown in figure 3-24 are usually provided for this purpose. Another type of ring compressing tool is a steel band that can be placed around the ring and tightened. PISTONS Trunk-type pistons are subject to forces such as gas pressure, side thrust, inertia, and friction. These forces, together with overheating and the presence of foreign matter, may cause troubles such as undue piston wear, crown and land dragging, cracks, piston seizure, clogged oil holes, and piston pin bushing wear. Excessive Piston-to-Liner Clearance Symptoms of excessive clearance between a piston and its cylinder are piston slap and excessive oil consumption. Piston slap occurs just after top dead center and bottom dead center, as the piston shifts its thrust from one side to the other. As the cylinder taper increases with wear, oil consumption increases. Since taper causes the rings to flex on each stroke of the piston, excess ring wear occurs, allowing lube oil to pass and 3-16