What Are Signs of a Blown Piston Ring?

by TJ Hinton

Each piston in your car's engine is equipped with two separate compression rings toward the crown of the piston, and an oil control ring assembly toward the skirt. The rings ride within annular grooves in the piston. The compression rings contain the pressure of the expanding gases within the combustion chamber, helping to harness the power being generated while preventing blow-by gases from entering the crankcase. The oil control ring assembly scrapes excess oil from the cylinder walls ahead of the compression rings to prevent the oil from entering the combustion chamber. A failure in any of these rings will result in a loss of performance coupled with other problems and symptoms.

Broken Compression Rings

The effect of a broken compression ring will immediately manifest itself in the form of loss of performance, rough idle and possibly a dead miss in the affected cylinder. Lack of containment of the combustion gases will cause blow-by gases to enter the crankcase and exit through the positive crankcase ventilation system. Your PCV valve will most likely be located on a valve cover. Disconnect the breather tube from the PCV, and if you notice a strong and smoky discharge from the valve, then chances are good that compression rings are broken. Besides the obvious performance problems, other problems can develop over time. For instance, a diesel engine that runs high-sulfur fuel, such as in farm or marine applications, can be severely damaged by a compression leak. Partially burned fuel blows by the rings, and the sulfur in the fuel mixes with water traces in the oil, and combines to form sulfuric acid, which will damage the internal components of the engine. In gasoline engines, the fuel acts as a solvent that thins the oil and prevents it from properly protecting the internals. Check the compression using a compression tester. Your compression typically should be around 160 to 180 psi, with no more than 15 percent variation between the cylinders. If the compression is low on one cylinder, you may have a broken ring on that cylinder.

Broken Oil Control Ring

A broken oil control ring assembly will be noticeable by the quality of the exhaust, which will turn blue and have a decidedly oily smell to it. The exhaust will emit a puff of blue smoke per revolution for the bad cylinder, and normal-looking exhaust for the good cylinders. These stacatto puffs makes it easy to diagnose visually. Other symptoms include oil loss in the absence of leaks, and oil fouling on the spark plug of the affected cylinder.

Mechanical Damage

Besides the damage caused by blow-by gases, improper lubrication and free hydrocarbons in the oil, there may be mechanical damage evident. The ends of the rings can gouge the cylinder wall, preventing the other rings from making good contact with the cylinder walls and exacerbating the symptoms. The annular ring grooves in the piston can be damaged, and since the cylinder walls and rings are both harder than the aluminum piston, the piston itself can be damaged or partially broken, leading to even greater damage. Since any broken pieces are likely to wind up in the bottom of the crankcase, possibly causing more damage, you should repair broken rings promptly. You can pull the head to inspect the cylinder walls for damage, or use a small mechanic's camera inserted through the spark plug hole for a less-invasive procedure.

Causes of Broken Rings

As long as the rings were properly sized and installed during engine assembly, any failure in the rings are most likely being caused by another mechanical problem. When an engine becomes overheated, for instance, the piston will expand, reducing the piston-to-cylinder clearance. This reduced clearance can cause a metal transfer from the piston to the cylinder wall called galling. The transferred aluminum can build up on the cylinder wall and cause the top compression ring to leak or break. Oil control rings can break if there is excessive piston-to-cylinder clearance allowing too much piston slap to occur. The skirt of the piston, and indeed the cylinder wall itself, can be damaged and this damage can in turn wipe out the oil control ring assembly.

About the Author

TJ Hinton trained as an auto mechanic at Mississippi Gulf Coast Community College and then later graduated from MMI as a certified motorcycle mechanic . He's also worked for 20+ years in home construction, remodeling and repair. His articles appear on InternetAutoGuide.com and TopSpeed.com.

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