LO 7-5 Identify and explain the purpose and functions of piston rings, their construction features, and their ring shapes according to ring function

Oil Control Rings
The last ring type found on a ring pack is the oil control ring. This ring is usually cast iron with scraper rails that remove excess oil from the cylinder wall while leaving a metered quantity of oil for lubrication. This oil film thickness is typically no more than 0.000004" (0.000102 mm) thick.

The oil control ring is usually a low-tension type of ring and has no significant gas pressure acting upon it. For this reason, a metal expander is placed behind the ring to maintain a constant, uniform pressure against the cylinder walls by the scraper rails Figure 37. The expander ring may be corrugated, slotted, or a wire coil Figure 38. Like the intermediate and compression rings, the oil control ring is vulnerable to premature wear from dirt ingestion into the engine. Scraper rails will show premature wear if abnormal amounts of dirt contaminate the oil through the intake air.

FIGURE 07-37: Oil control rings consists of two scraper rails plus an expander ring. This coiled expander maintains consistent ring pressure against the cylinder wall.

FIGURE 07-38: A corrugated ring expander helps maintain consistent scraper ring pressure against the cylinder walls, since less gas pressure is available to help ring sealing.