What is Piston Ring?- Function, Types, and Uses

An engine is a simple mechanical device, an air pump really, that uses pistons and valves to pull air in and push air out. The pistons in an engine block must be sealed inside each cylinder in order for the process to be efficient.

Because the pistons travel through the stroke millions of times throughout the life of the engine, the seals are critical. This is done with a series of rings that go around the top half of the piston, sealing it to the cylinder wall. When the piston rings wear out, the engine’s efficiency suffers.

What is Piston Ring?

A piston ring is a metallic split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine.

The three main functions of piston rings in reciprocating engines are:

  • Sealing the combustion chamber so that there is no transfer of gases from the combustion chamber to the crank.
  • Supporting heat transfer from the piston to the cylinder wall.
  • Regulating engine oil consumption.

Piston rings are commonly made from cast iron or steel. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces.

Piston rings are made to maintain the cylinder and combustion pressure of the automobile. They prevent any oil from seeping into the combustion chamber as well as sealing in the air and fuel to be able to compress them. There are typically three piston rings to every piston, and each one of these rings does different things to help the engine operate.

Piston rings are a major source of hints to identify if the engine is two-stroke or four-stroke. Three piston rings suggest that it is a four-stroke engine while two piston rings suggest that it is a two-stroke engine.

The function of the Piston Ring

The main functions of piston rings in an engine are:

  • Sealing the combustion chamber so that there is minimal loss of gases to the crankcase.
  • Improving heat transfer from the piston to the cylinder wall.
  • Maintaining the proper quantity of oil between the piston and the cylinder wall
  • Regulating engine oil consumption by scraping oil from the cylinder walls back to the sump.

Types of Piston Rings

Piston rings commonly used on small engines include the three types of piston rings as mentioned below:

  • Compression Ring
  • Wiper Ring
  • Oil Ring
Piston Rings

1. Compression Ring

The compression ring is the topmost ring in the piston attached to its outside diameter. The main function of the compression ring is to seal the gap between the piston and the cylinder walls.

Sealing this gap with the outer diameter of the piston and the cylinder walls ensure that the air-fuel mixture in the combustion engine does not move down to the crankcase and causes low compression and power.

In addition, this sealing also makes sure that the engine oil in the crankcase, used for lubrication, does not move up into the combustion chamber and gets burnt.

Engine oil moving into the combustion chamber and getting burnt will effectively result in excessive consumption of oil in the engine and then to low engine oil levels.

So, essentially the compression piston ring ensures that the combustion chamber and the crankcase are segregated.

The air-fuel mixture in the combustion chamber is not allowed to trickle down into the crankcase and the engine oil is not allowed to move up into the combustion chamber.

The advantages of a compression ring sealing the gap between the piston and the cylinder walls include maintaining high compression in the combustion chamber, high power and acceleration in the vehicle, and no unnecessary burning of engine oil.

2. Wiper Ring

The wiper ring also called a Napier ring, or backup compression ring, are installed below the compression ring. Their main function is to clean the liner surface off the excess oil and to act as support backup ring on stopping any gas leakage further down which escaped the top compression ring.

Most of the wiper rings have a taper angle face which is positioned toward the bottom to provide a wiping action as the piston moves toward the crankshaft.

If the wiper ring is incorrectly installed with the tapered angle closest to the compression ring, it results in excessive oil consumption. This is caused by the wiper ring wiping excess oil toward the combustion chamber.

3. Oil Ring

An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block.

Two-stroke cycle engines do not require oil rings because lubrication is supplied by mixing oil in the gasoline, and an oil reservoir is not required.

An oil ring includes two thin rails or running surfaces. Holes or slots cut into the radial center of the ring allow the flow of excess oil back to the oil reservoir. Oil rings are commonly one piece, incorporating all of these features.

Some on-piece oil rings utilize a spring expander to apply additional radial pressure to the piston ring. This increases the unit (measured amount of force and running surface size) pressure applied at the cylinder wall.

The oil ring has the highest inherent pressure of the three rings on the piston. Some Briggs & Stratton engines use a tree-piece oil ring consisting of two rails and an expander. The oil rings are located on each side of the expander.

The expander usually contains multiple slots or windows to return oil to the piston ring groove. The oil ring uses inherent piston ring pressure, expander pressure, and the high unit pressure provided by the small running surface of the thin rails.

Symptoms Of Bad Piston Rings

Symptoms of bad piston rings are similar to the symptoms of worn or damaged valve seals. Catching symptoms in the early stages will save your engine and save you from a hefty bill. Here is a list of the most common symptoms for bad piston rings:

  • White or gray exhaust smoke
  • Excessive oil consumption
  • Low power for acceleration
  • Overall loss of power or poor performance
  • Oil in the intake manifold

1. White or Gray Exhaust Smoke.

If there is a lot of exhaust smoke coming out of your vehicle, this could be an easy sign that you have bad piston rings. This smoke will look very thick and have dark gray and blue colors to it. It is often accompanied with a burning oil smell.

When you have bad piston rings, engine oil will start leaking into the combustion chamber. Once that happens, the oil will burn and create the thick blue gray exhaust smoke that you see coming out.

2. Excessive Oil Consumption.

The moment your check engine light turns on because of excess oil consumption, it is highly likely that your piston rings need replacement. Worn-out piston rings will lead to oil leaks in the combustion chamber which automatically will manifest themselves as more oil consumption by the vehicle. So check the Engine oil Level Regularly.

Therefore, your car will require you to regularly add oil more frequently than the average 4000 miles before refill. Immediately inspect your piston rings when your car begins consuming more than the regular amount of oil.

3. Overall loss of power or poor performance.

The worst-case symptom of bad piston rings is when you lose all power in your vehicle. You won’t be able to accelerate your vehicle at all and its overall performance will be very poor.

You’ll probably have to tow your vehicle to the nearest mechanic because the performance will be so bad. However, you will only experience this symptom if you’ve ignored the first three earlier symptoms as they occurred.

4. Low Power for Acceleration.

Technically, this symptom is a part of declining engine performance. However, the engine will often start losing acceleration before it begins to shudder or stutter. With a loss of acceleration, you might notice difficulty getting your car to smoothly and quickly accelerate to higher speeds.

Also, a vehicle with poor acceleration will struggle to pull itself up hills. Luckily, this issue does not cause long-term damage and has an easy fix. Changing the piston rings will immediately restore the standard acceleration rate.

5. Oil in the intake manifold.

Inside your vehicle’s engine bay is a small filter where the intake pulls in air. This air mixes with the fuel in the combustion cylinder and is partially responsible for turning the engine. The fuel won’t ignite without air, and the engine won’t turn over.

When combustion happens around failed piston rings, it sometimes forces oil back into the intake manifold. This process is known as “blow-by.” You will identify blow-by with an oily film in the intake or oil drops on the air filter.

If it sounds serious, it’s because it is serious. Oil does not belong in the intake manifold, and it’s hazardous when it shows up here. Any static shock can ignite the oil, seriously damaging your engine or harming you.

Piston Rings Replacement Cost

Fixing the pistons or replacing piston rings is pretty expensive—a safe estimate is at least $1000, but prices can range as high as $5000 depending on your mechanic and how many need to be replaced.

The average cost for piston ring replacement is $75-$3,500, piston rings themselves aren’t all that costly, averaging out to around $100, but the labor cost is steep due to how intricate and complicated the job of replacing them is.

A mechanic will have to totally disassemble your engine to get to the pistons, after which the engine will have to be reassembled.

A mechanic could charge you between $85 and $300 per hour in labor expenses alone, depending on their experience and the shop itself. This adds up quickly because even a seasoned mechanic takes upwards of 10 hours to complete a piston ring replacement.

On the bright side, your car will be back in working order, and you probably won’t need to have your pistons fixed again for a long while.


Which Piston Ring Prevents Combustion Pressure from Entering The Crankcase?

Most pistons use three rings, two upper compression rings, and one oil ring on the bottom. The compression rings prevent blow-by (combustion pressure leaking into the engine crankcase). The oil rings prevent oil from entering the combustion chamber.

Which Piston Ring Is Located In The Middle Of The Engine?

A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process.

How To Clean Piston Ring Grooves?

The first thing you need to do is get a piston ring groove cleaner. Then remove the piston rings. You will need to do this carefully not to damage the piston. Use a seafoam spray and ensure it gets in the piston ring groove. Set the piston ring groove cleaner and slide it up with the cutting edge.
You can then spin the piston ring groove cleaner to clean the groove’s carbon. Apart from the carbon, it will also clean out any rust or sediment accumulated in the piston ring groove. Do this until you start seeing shining metal; you can now stop. If you continue, you will be cutting through the piston, thus damaging it.

Which Tool Is Best For Measuring Piston Ring End Gap And Valve Clearance?

The feeler gauge is suitable for measuring the valve clearance, ring joint clearance on piston rings and for many other applications.

What happens when piston rings go bad?

Worn piston rings can also cause your engine to experience a reduction in power. There will be less compression, so your engine may struggle to accelerate as effectively. If you’re using the throttle and waiting increasingly long periods of time for your engine to speed up, then the piston rings may be at fault.