Although oil rings come in a large variety of designs, they must all do two things: (1) distribute enough oil to the cylinder wall to prevent metal-to- metal contact, and (2) control the amount of oil distributed.

Without an adequate oil film between the rings and the cylinder, undue friction occurs, resulting in excessive wear of the rings and the cylinder wall. On the other hand, too much oil is as undesirable as not enough oil. If too much oil is distributed by the rings, the oil may reach the combustion space and burn, wasting oil and causing smoky exhaust and excessive carbon deposits in the cylinder. Such carbon deposits may cause the rings to stick in their grooves. Sticking rings lead to a poor gas seal. Thus, oil rings provide an important function in proper control and distribution of the lubricating oil. Some types of oil rings are shown in views C, D, and E of figure 4-13.

Different manufacturers use a variety of terms in their technical manuals to identify the oil rings of an enginesuch terms as oil control, oil scraper, oil wiper, oil cutter, oil drain, and oil regulating. Regardless of the identifying terms used, all such rings are used to limit the oil film on the cylinder walls and to provide adequate lubrication to the compression rings.

Most oil control rings use some type of expander to force them against the cylinder wall. This aids in wiping the excess oil from the cylinder wall. For example, a General Motors 6-71 piston has two sets of oil control rings placed on the skirt below the piston pin. Both sets are identical, each consisting of three pieces (two rings and an expander). (See fig. 4-12.) The ring illustrated in view E of figure 4-13 is also a three-piece oil ring. In rings of this type, the two scraping pieces have very narrow faces bearing on the cylinder wall, which permit the ring assembly to conform rapidly to the shape of the cylinder wall. Since the ring tension is concentrated on a small area, the rings will cut through the oil film easily and remove the excess oil. The bevel on the upper edge of each ring face causes the ring to ride over the oil film as the piston moves toward top dead center (TDC), but as the piston moves downward for intake and power, the sharp, hook-like lower edge of each ring scrapes or wipes the oil from the cylinder wall.

Another example of differences in terminology and location is found in the Fairbanks-Morse (FM) 38D8 1/8. A piston in this type of engine has three oil rings all located on the skirt end. The two nearest the crankshaft end of the piston are called oil drain rings, while the ring nearest the pin bosses is referred to as the scraper. The drain rings are slotted to permit oil to pass through the ring and to continue on through the holes drilled in the ring grooves. View D of figure 4-13 shows one type of slotted oil ring. Additional informa-tion concerning pistons and piston rings can be found in Naval Ships Technical Manual, chapter 233.