may be one or more oil control rings per piston; two rings  may be installed in the same groove, or they may be installed  in separate grooves. Oil control rings regulate the thickness  of the oil film on the cylinder wall. If too much oil enters the  combustion chamber, it burns and leaves a thick coating of  carbon on the combustion chamber walls, the piston head, the  spark plugs, and the valve heads. This carbon can cause the  valves and piston rings to stick if it enters the ring grooves  or valve guides. In addition, the carbon can cause spark plug  misfiring as well as detonation, preignition, or excessive  oil consumption. To allow the surplus oil to return to the  crankcase, holes are drilled in the bottom of the oil control  piston ring grooves or in the lands next to these grooves. the relative motions of the piston and the articulated rod and  that of the human arm. The piston pin used in modern aircraft  engines is the full-floating type, so called because the pin is  free to rotate in both the piston and in the connecting rod  piston-pin bearing. The piston pin must be held in place to  prevent the pin ends from scoring the cylinder walls. A plug  of relatively soft aluminum in the pin end provides a good  bearing surface against the cylinder wall. The piston rings prevent leakage of gas pressure from the  combustion chamber and reduce to a minimum the seepage  of oil into the combustion chamber.  [Figure 1-15]  The rings  fit into the piston grooves but spring out to press against the  cylinder walls; when properly lubricated, the rings form an  effective gas seal. The oil scraper ring usually has a beveled face and is installed  in the groove at the bottom of the piston skirt. The ring is  installed with the scraping edge away from the piston head  or in the reverse position, depending upon cylinder position  and the engine series. In the reverse position, the scraper ring  retains the surplus oil above the ring on the upward piston  stroke, and this oil is returned to the crankcase by the oil  control rings on the downward stroke. Most piston rings are made of high-grade cast iron.  [Figure 1-14]  After the rings are made, they are ground to  the cross-section desired. Then they are split so that they can  be slipped over the outside of the piston and into the ring  grooves that are machined in the piston wall. Since their  purpose is to seal the clearance between the piston and the  cylinder wall, they must fit the cylinder wall snugly enough  to provide a gastight fit. They must exert equal pressure at  all points on the cylinder wall and must make a gastight fit  against the sides of the ring grooves. The portion of the engine in which the power is developed  is called the cylinder.  [Figure 1-16]  The cylinder provides  a combustion chamber where the burning and expansion of  Gray cast iron is most often used in making piston rings. In  some engines, chrome-plated mild steel piston rings are used  in the top compression ring groove because these rings can  better withstand the high temperatures present at this point.  Chrome rings must be used with steel cylinder walls. Never  use chrome rings on chrome cylinders.  The purpose of the compression rings is to prevent the escape  of combustion gases past the piston during engine operation.  They are placed in the ring grooves immediately below the  piston head. The number of compression rings used on each  piston is determined by the type of engine and its design,  although most aircraft engines use two compression rings  plus one or more oil control rings. The cross-section of the ring is either rectangular or wedge  shaped with a tapered face. The tapered face presents a  narrow bearing edge to the cylinder wall, which helps to  reduce friction and provide better sealing. Oil control rings are placed in the grooves immediately below  the compression rings and above the piston pin bores. There  1-14 Figure 1-16. An example of an engine cylinder.  Oil Control Rings Compression Ring Cylinders Piston Ring Construction  Oil Scraper Ring Piston Rings