Piston Rings: Their Functions and Types

A piston ring is a type of expandable, metallic split ring attached to the outer diameter of a piston within an internal combustion engine. Piston rings are manufactured and classified depending on their function and usability. They are mainly used in the combustion chamber of two or four stroke engines, though marine engines can have three or more different types of rings fitted on the circumference of the pistons. This blog will further discuss the functions of piston rings and their different types.

The piston ring is an essential piston part, and its functionality differs greatly depending on the type and size of engine it is used in. Piston rings have four main functions within an engine. They are:

  1. To seal the combustion chamber so as to minimize loss of gases to the crankcase.
  2. To improve the efficiency of the transfer of heat from the piston to the cylinder wall.
  3. To maintain the appropriate amount of oil between the piston and the cylinder wall.
  4. To regulate engine oil consumption by scraping oil from the cylinder walls back to the sump.

As piston rings serve multiple functions, there are multiple types. The most common types of piston rings are compression rings, wiper rings, and oil rings.

  • Compression rings or pressure rings are the topmost ring, closest to the combustion gases. As such, they are exposed to the most chemical corrosion and the highest operating temperatures. Compression rings transfer roughly 70% of the combustion chamber heat from the piston to the cylinder wall. There are two subtypes of compression rings: taper-faced or barrel-faced. A taper faced compression ring is a piston ring with a 1° taper angle on its running surface that provides a slight wiping action to keep excess oil from reaching the combustion chamber. Barrel-faced compression rings feature a curved running surface to provide constant lubrication of the ring and cylinder wall. The curved surface also creates a wedge effect to optimize oil distribution throughout the piston. Furthermore, it reduces the possibility of an oil film breakdown due to excessive pressure or piston tilt during operation.

  • The wiper ring, also called the scraper or Napier ring, is the second-closest ring to the cylinder head. Wiper rings are used to provide a constant thickness of oil film to lubricate the surface of the above compression ring. Most wiper rings feature a taper angle face positioned toward the oil reservoir that provides a wiping action as the piston moves toward the crankshaft. The angled face helps lead excess oil from the cylinder wall back to the ring and eventually back to the oil reservoir.

  • Lastly, oil rings include two thin rails known as running surfaces. Holes or slots cut into the radial center of the ring allow excess oil to flow back into the oil reservoir. Oil rings are most commonly a single piece that incorporates all of these features, though certain one-piece oil rings use an expander to provide additional radial pressure to the piston ring. However, some engines utilize a three-piece oil ring that comprises two rails and an expander. The expander contains multiple holes to return oil to the piston ring groove. Of the three rings on a piston, the oil ring has the highest inherent pressure.


For all types of piston plane rings and much more, look no further than NSN Supplies, a trusted supplier of all kinds of aircraft parts. Owned and operated by ASAP Semiconductor, we are an online distributor of aviation components as well as parts pertaining to the aerospace, civil aircraft, defense, industrial, and electronics markets. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at 1-505-365-1770 or email us at sales@nsn-supplies.com.


February 2, 2021

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