Both Sealing and Lubrication are Important. Piston Rings that Wear and Loss of Tension Cause Oil to Rise.

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For an internal combustion engine, the explosive pressure generated when the compressed air mixture burns in the combustion chamber pushes the piston down and turns the crankshaft, so the key is how to keep the pressure from escaping, and the piston ring is there for that purpose. Here we will reconfirm the importance of piston rings through one of the problems that can occur due to piston ring failure: oil rises.

Piston rings need to keep compression without friction loss.

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When the piston is pulled out of the cylinder with the oil up problem, the oil ring is exposed but remains almost the same diameter as the piston and has no tension at all. This means that engine oil cannot be scraped off the inside wall of the cylinder and enters the combustion chamber through the gap between the second ring and the top ring.

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Top and second rings that protrude well out of the piston ring groove = firm tension.
The oil ring is sandwiched between corrugated spacers and upper and lower side rails, and it is correct that the side rails have enough tension to protrude from the ring groove as well as the top and second rings. The side rails of this piston do not protrude from the ring groove, which means that the oil film in the cylinder cannot be controlled. Although it is a very rare occurrence, a drop in tension, apart from ring cracks and wear, can cause oil to rise.

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The vehicle that caused the oil to rise was an overseas production model and genuine parts were not available, so we purchased a third-party set based on the bore diameter and piston ring dimensions. We were lucky that it worked by chance, but if rings that fit the pistons are not available, we may be able to find a piston and ring set for another model that is available.

For an engine to run smoothly, there must be adequate clearance in each part. If we focus on the cylinder and piston, we find that clearance of 3/100 to 5/100 mm is provided, depending on the engine specifications. This clearance is necessary to prevent biting even if each part expands thermally during engine operation.

On top of that, the piston rings are sliding in close contact with the cylinder to prevent leakage of the pressure generated by the combustion of the air mixture. As any rider interested in engines knows, there are three types of piston rings set in a typical 4-stroke engine. The top ring prevents compression pressure from escaping through the gap between the piston and cylinder, while the second ring and oil ring adjust the oil film necessary for lubrication.
The top ring and second ring also conduct the heat received by the piston during the combustion of the cylinder.

Ideally, the gap between the piston and piston ring and between the piston ring and cylinder should be as small as possible to make the piston and cylinder airtight, but a proper oil film is essential because direct metal-to-metal contact and sliding can cause burning. Outward tension is also necessary to ensure that the ring is pressed evenly against the cylinder wall. When a piston with a piston ring installed is inserted into a cylinder, you can feel the metal of the ring and cylinder rubbing against each other, but during actual operation, there is an oil film of engine oil between the piston and cylinder, so there is no seizure.

Seizure is a concern, but this does not mean that the piston ring tension should be set low so that more engine oil comes in contact with it. If too much oil adheres to the cylinder wall, it will remain in the combustion chamber and burn with the mixture when it burns.

Unlike oil for 2-stroke engines, oil for 4-stroke engines is not designed to burn, so it can adhere to pistons and piston rings as carbon deposits and cause white smoke to be released from the exhaust system. It also weakens the tension of the piston rings, allowing compression pressure to pass through the gap between the rings and cylinder to the crankcase side, causing blow-by gases.

POINT

Point 1 - Piston rings have important roles in maintaining compression pressure, conducting piston heat, and managing oil film on lubricated surfaces.

Point 2 - Piston rings have evolved over time to achieve both airtightness that does not leak compression pressure as blow-by gas and tension that does not cause friction loss.

Enlarged mating gaps and reduced tension are the main causes of oil rises.

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Oil ring spacer shapes vary by piston ring manufacturer and model. The thin side rails between the top and bottom of the spacer tilt slightly like a valve in conjunction with the piston stroke, allowing excess oil adhering to the cylinder's inner wall to flow toward the spacer and through a hole in the bottom of the piston ring into the crankcase.

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The new oil ring has side rails that extend well beyond the ring groove and must be gripped with a finger to shrink to the piston diameter. This should prevent oil from rising.

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When assembling a cylinder with new piston rings with high tension, it is important to fit the rings in the groove so that they do not break at the end of the sleeve. You can hold them with your fingers, but here we used a band-type piston ring compressor.

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Curing the piston pin with a piece of wood to prevent it from tilting on its fulcrum, the cylinder is placed over the piston. With the band of the compressor holding the entire circumference of the piston ring, the ring can be inserted without getting caught in the sleeve by pushing the band down with the lower end of the cylinder.

The tension required for piston rings has been studied by piston ring manufacturers for many years and is still evolving to improve fuel efficiency, environmental measures, and engine performance. Prior to the 1960s, piston rings were generally made of cast iron and one-piece oil rings were commonly used, but since the 1970s, the material has become steel and three-piece oil rings combined with side rails and spacers have become popular.

The steel ring is thinner and has higher tensile strength than cast iron rings, increasing surface pressure at the cylinder contact area and reducing friction loss, and the three-piece design effectively scrapes engine oil of the cylinder wall.

Even though the piston rings and cylinder are lubricated with engine oil, this does not necessarily mean that the piston ring performance is permanent. If the engine oil is not changed for a long period of time, the viscosity and strength of the oil film will decrease, and even if it does not lead to seizure, the ring surface may become worn, causing the gap between the rings to widen. The surface of the top ring is coated to improve wear resistance, but scratches on the surface can reduce compression pressure retention and cause increased blow-by gas emissions.

In addition, to wear, piston rings also have the problem of reduced tension. When a piston ring is set on a piston, it protrudes outward from the ring groove and must be retracted with fingers or a piston ring compressor when inserted into the cylinder. This tension helps to maintain pressure in the cylinder, and if the tension drops for any reason, a problem will occur.

One of the most troublesome problems is "oil rises" caused by low oil ring tension. However, when the tension of the oil ring, which plays the main role in scraping off the engine oil from the cylinder walls, is lowered, the oil enters the combustion chamber and burns together with the combustion mixture, causing white smoke to be emitted from the exhaust system. The oil rings are the main mechanism by which engine oil enters the combustion chambers. Another cause of engine oil entering the combustion chamber is oil drop that originates from intake valve stem seal wear. Generally, white smoke at startup indicates low oil, while a high oil level at high rpm or high load can be judged as a high oil level.

In the case of oil rises, there are cases where the symptoms can be temporarily alleviated with chemicals that improve sealing properties, but fundamentally, the only way is to replace the piston rings with oil rings, which is the theory that a set of three rings should be replaced even if the cause is the piston ring oil rings. If the wear is caused by poor oil management, the cause lies with the rider, but in a rare case, the rings themselves may be hit by rings with reduced tension due to the material, manufacturing method, or production lot of the rings. This can only be described as bad luck.

Also, when a long-term immobilized motorcycle is restarted after a long time, white smoke may subside after symptoms such as a temporary oil rupture. One possible cause is oil-induced ring sticking. When the motorcycle is immobile for a long time, the oil causes the piston rings to stick in the piston ring grooves and temporarily lose their airtightness, resulting in oil rises. However, the temperature of the piston and rings rises, the viscosity of the oil decreases, and furthermore, new oil may be circulated and cleaned to allow it to move freely and tension may be restored. After a long period of immobility, it is recommended to change the engine oil after using flushing oil with cleaning action and see how it goes.

It is not easy for a rider to know if white smoke is coming from the exhaust system of his own motorcycle while riding.
This is especially true in the early stages of high oil content when only a small amount of white smoke is visible in the rearview mirror. However, when riding at night, the white smoke may appear foggy when illuminated by the headlights of following vehicles, so if you notice anything unusual, stop the bike, put it in neutral, and open the throttle wide to check for symptoms of high oil pressure.

POINT

Point 1 - Oil rises as engine oil travels between the cylinder and piston into the combustion chamber, often caused by piston rings. 

Point 2 - Oil rises due to low piston ring tension can occur even at low mileage.

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