Piston Rings: How to Choose the Best One for Your Needs

Engine Piston Rings

Piston Rings are a necessary part of any engine. They are used to increase the engine’s power and change the engine’s rpm. Piston rings are also used to control the air/fuel mixture in the engine. The use of piston rings is important because it can affect the performance of the engine and the gas mileage of the car. Piston Rings are a necessary part of any engine.

What is Diesel Engine Piston Rings?

A piston ring is a split ring that can expand and provides a seal between the cylinder wall and piston.

Engine Piston Rings perform the following functions.

1. Sealing of the combustion chamber by outward expansion due to the gas pressure acting behind them.
2. Prevents the passing of gases and combustion products.
3. Prevention of the gases and combustion products passing the piston.
4. It also conducts heat away from the piston to the liner.
5. It also controls the cylinder oil.

Engine Piston Rings material

Most commonly Cast iron is used for the manufacturing of Piston rings. Heat, load, and various other forces don’t affect the cast iron to lose its original shape.

Facts about Engine Piston Rings

Engine design and material of cylinder liner will decide the size and composition of the piston ring.

The effectiveness of a piston ring for leak tightness and sealing decides the overhaul period of a cylinder.

The output generation of an engine depends upon the function of a piston ring.

The piston ring surface should have complete contact with the liner surface and its complete depth and periphery.

The faulty function of the piston ring is the origination of troubles in Diesel engines.

Types of Engine Piston Rings

1. Top Piston Ring/Compression ring,
2. Wiper ring/2^nd ring
3. Oil ring.

Top Piston Ring/Compression ring

The compression ring/Fire ring fits in the top groove of the piston nearest to the piston crown. The function of the top ring is to provide the sealing to the combustion chamber and prevents the leakage of gases during the combustion process.

The pressure of the combustion gases during the ignition of the air-fuel mixture in power stroke acts on the piston head and forces the piston downward.

Gases at pressure enter the piston ring groove through the gap between the cylinder liner wall and the piston. This combustion gas pressure forces the piston towards the cylinder wall and forms the sealing. The gas pressure on the piston ring is equal to the combustion gas pressure.

The compression ring has direct contact with the combustion gases and has direct exposure to the maximum operating temperature and the acidic corrosion of the gases due to sulfur.

The top ring/ Compression ring transfers about 70 to 75 % of the heat from the piston to the cylinder liner.

Sealing of combustion chamber by a compression ring is through the following forces.

1. Inherent Pressure;-  Inherent pressure is the tangential force due to the springing action of the ring that expands the ring outward, due to the material properties and the design of the ring. The ring-free gap will determine the inherent pressure. Greater the free ring gap more the inherent pressure. A free gap of a piston ring is the gap between the two ends of a piston ring in an uncompressed position.
2. Applied pressure.;-  Applied pressure is due to the application of pressure by combustion gases on the piston ring.

Piston ring Designs

1. Taper-faced compression ring;- This design of ring has a taper angle of approximately 10 on the running surface  The purpose of providing the taper is to have a mild wiping action to avoid the excess lubricating oil passing to the combustion chamber.
2. Compression ring with Barrel face; – Surface with curved running on the ring provides the positive lubrication between the piston ring and the cylinder liner wall. This ring also has a wedge effect to ensure the proper oil distribution throughout the piston stroke. The curved surface reduces the possibility of oil film breakage due to the pressure on the ring by tilting the action of the piston.

Wiper Ring/Scraper ring/2^nd ring

This piston ring with the running tapered face has the placement in the 2nd ring groove between the compression ring and the oil ring.

This ring supports the top ring in sealing the combustion gases as well as controls the oil. The tangential force of the piston ring holds the ring against the cylinder wall. The ring has an undercut at the bottom side which scrapes the oil down to the engine sump. This ring also provides the constant oil film for lubrication on the running surfaces of the compression ring.

The wrong installation of the wiper ring will cause excessive lube oil consumption since it will wipe up the oil towards the combustion chamber.

Oil scraper Ring

The placement of this ring is in the groove below the 2nd groove near the crankcase. This ring wipes the excess lubricating oil from the cylinder wall to the sump in downward movement of the piston. The oil returns back to the sump via ring-opening to the engine sump.

An oil ring consists of two thin operating surfaces. Slots cut into the center will lead the excess oil back into the oil sump. All the features are available in a single-piece oil ring. Every oil ring has an expander for the additional radial pressure on the ring. This helps in keeping the ring in contact with the cylinder wall. Oil rings are with the highest inherent pressure of the three rings in the piston.

 Oil ring has the three pressures as below.

1. Piston ring pressure
2. Expander pressure
3. High pressure by the running surface of the thin rails.

During the manufacturing of a ring, a gap is cut on the machined outring from a casting drum. The free width of the gap is as per the pre-calculated dimensions to meet the working clearance.

It is not possible to design a complete round ring. It is general practice to coat for the running-in process. The ring achieves a round shape only on completion of the running-in period. After running for the period the ring will efficiently seal the surface.

Normally rings have two coatings. The outer coating will be of soft aluminum bronze to run in and proper bedding of the ring in a short period. The secondary coat is hard and scuffs to provide the strength to ring for long-lasting life. The hard coat increases the time between overhauls.

Type of joint in Diesel Engine Piston Rings

As per the above fig types of joints for piston rings are as under.

1. A is the plain joint
2. B & C are the bevel joints
3. D is the stepped or overlaps joint.

Due to the working of a ring under the severe conditions of pressure, heat and sudden reversal the suitable and most popular rings are of A and B (C) types.

 The ring of D design is costly but has the best sealing effects. Apart from the high cost, the overlap joint type rings have a serious drawback of breaking due to thin ends. Many cylinder liners had serious damage due to the broken piece entering the working surface and causing scuffing.

Type of Diesel Engine Piston Rings Clearances

An important point to be given consideration is that the piston expands outward and the piston ring expands inward. Both the deformations will tend to decrease the clearance between the bottom of the grove and the inner surface of the piston rings.

Another point is that in a few cases piston crown expands in a way that the width of the upper ring groove will be reduced considerably.

Butt clearance of Diesel Engine Piston Rings

1. it is the cut that is made in the ring such that due to thermal expansion the ring doesn’t collapse. Maintain the butt clearance in the range of 0.4 to0.5% of the cylinder liner’s internal diameter.

It has high importance for the following reasons.

•  If all the butts get aligned the combustion gases blow by or blow pass takes place which might lead to scavenging fire.
• If the butt clearance is less than the specified limit, the end of the piston ring will touch due to the thermal expansion. The ring will expand against the cylinder wall and will increase the friction.

The increase in friction will result in the breakage of the piston ring.

Axial clearance

it is the vertical distance between the piston ring and piston ring groove.

Importance of axial clearance in Diesel Engine Piston Rings

1. It allows pressure to build up behind the ring  

2. it allows oil to flow into the groove

3. It allows pressure to build up behind the ring.

Vertical Clearance of Diesel Engine Piston Rings

 A new set of piston and piston rings will have the normal vertical clearance. After a prolonged run, the vertical clearance will increase due to the wear down in the ring groove and piston rings. Inadequate clearance will cause the following.

1. It restricts free movement during operation.
2. Vertical clearance less than the specified limit will cause rings to stick in the groove and break as they come up to operating temperature.
3. Excessive clearance of the ring will cause hammering and ring groove wear.

Piston Ring Clearances measurement Procedure.

To measure butt clearance insert the rings into the bottom of the liner and ensure that they are in line with the linear axis.

Measure the joint of the piston ring with the help of a slip gauge or feeler gauge for the exact measurement.

Measure the clearance in four positions at 12^o clock, 3^o clock, 6^o clock, and 9^o clock positions.

Axial clearance is the clearance from the top of the ring to the top of ring groove.

Combustion gases cannot flow behind the rings in case the clearance is less than the minimum limit. To measure axial clearance feed the ring in the respective groove such that it rests on the lower surface of the groove.

Use the feeler gauge to measure the readings between the upper face of the ring groove and the piston ring.

Take the readings in four positions at 12^o clock, 3^o Clock, 6^o clock, and 9^o clock positions.

The clearance between the inner diameter of the ring groove and the inner diameter of the piston ring is known as the back clearance.

The thickness of the ring should be less than the groove depth so that the outer ring face is inside the ring groove.

Measure the depth of the ring groove for the measurement of back clearance.

Use a Vernier caliper to measure the thickness of the ring.

 Take both these measurements in at least six different positions around the groove circumference. Back clearance will be a difference between these two readings. Replace the rings in case the back clearance is more than the limits.

 Measure the free gap of all the piston rings before dismantling the rings.

Diesel Engine Piston Rings clearance calculation formulae

Important checks before the fitment of Diesel Engine Piston Rings

For the achievement of the best results of Diesel Engine Piston Rings, carry out the following points. These points are in the light of past experience.

1. Try out the ring in the groove for which it is meant and ensure that it has the required clearance in the vertical direction, and is free to contact at the back and bottom of the groove.
2. Measure the butt clearance at the smallest diameter of the cylinder liner.
3. Remove the ridge from the cylinder liner top reversal point of the piston ring. A new ring may foul with this ridge and is likely to damage the cylinder liner with a heavy knocking sound.
4. Remove any ridge formation in the groove which may foul the top ring and create big damage to the piston ring groove.
5. Always follow the manufacturer’s recommendations on the fitment of the rings. Ensure the correct upward and downward position of the ring, failing to do so will have serious consequences of high lubricating oil consumption and loss of compression pressure.
6. Before the fitment of the ring with coil spring-loaded expander carry out the following.
7. Dismantle the coil spring.
8. Assemble the coil spring in the groove.
9. Assemble the ring in such a way that the ring joint is approximately 180 ⁰ offset to the spring joint.
10. To prevent the leakage from the coinciding ring joints, offset the ends of the adjacent rings by 180 ⁰ opposite to each other before springing in.

Our top picks on Engine Piston Rings

Mahle 41859CP Moly Ring Set

The Mahle 41859CP Moly Ring Set is the highest quality OE replacement piston ring set available. The Mahle replacement ring sets are designed with OE form, fit, and function. The ring set is made from high alloy steel base material and combined to make the ultimate piston ring material. The ring set is designed to be a high-quality replacement and offers a higher tensile and yield strength. The ring set is also designed to be extended fatigue to fit larger rings.

Sealed Power Standard Piston Ring Set

The Sealed Power Standard Piston Ring Set is a high-quality, unrivaled product that you can trust. It uses leading-edge materials and state-of-the-art manufacturing to produce rings that are precise and exact to your specifications. The standard tension feature ensures that the rings are able to handle the most demanding applications, and the drop-in design makes it easy to add or remove rings without having to remove the entire ring set. The economy design also reduces the cost of this product.

Sealed Power Premium Piston Ring Set

The Sealed Power Piston Ring Set is a unique and innovative product that is specifically designed for the automotive market. The rings are coated with the latest generation Plasma-Moly, which reduces friction and is very resistant to scuffing. The exacting tolerances ensure that the rings are the perfect fit for your vehicle. The fit type is vehicle Specific, which gives you the freedom to choose the perfect ring for your ride.

Stens 500-736 Piston Rings STD

The Stens 500-736 Piston Rings STD is a replacement for OEM parts for models K301 and K532. The Stens 500-736 Piston Rings STD are a replacement for OEM parts for models K301 and K532 and are designed to fit models K301 and K532 before serial number 1730100013. The Stens 500-736 Piston Rings STD are a replacement for OEM parts and are designed to fit models K301 and K532 and are the perfect size to replace OEM parts.

NR Racing Top Piston and Rings

This is a brand new, high-quality 70mm Flattop Piston and Rings for your outdoor power equipment. This is a great replacement for your worn or damaged rings. This part is designed for use with a standard .5-in.-diameter air-cooled Piston. The rings are made from high quality, durable aluminum and have a standard bore of .570. They will fit most standard air-cooled power equipment.

GOOFIT 54mm Piston Ring

The GOOFIT 54mm Piston Ring Kit Replacement for 125cc Horizontal Engine ATV Dirt Bike is designed to fit a variety of unlisted brand motorcycles. It features a 54mm piston diameter and 14mm piston pin diameter, making it the perfect replacement for SSR BMS NST JCL Taotao Baja Kazuma Sunl Coolster Jetmoto and other unlisted brand motorcycles. With a warranty, you can be sure that your motorcycle will be fixed or replaced if it fails.

FAQ on Piston Rings

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