The piston rings move back and forth with the piston, rubbing against the cylinder and receiving the combustion pressure without leaking. In order to keep the pressure from leaking out, it is essential that the gap between the rings is within the specified range, but at the same time, it is also important that the rings themselves are held tight against the cylinder. It is important to note that some of the cast rings that were used in many old and out-of-print models have lost their tension over time.
Piston ring mating gap is essential
Introduced in 1973 with 50cc and 80cc engines with 2-speed automatic transmissions, the Yamaha Chappy became popular as a family motorcycle and was later added to the lineup with a hand clutch version with a 4-speed transmission and a centrifugal clutch version with a 3-speed transmission. This engine is a 50cc with a centrifugal clutch and is unrestored with only 5000km in 40 years.
There was no noise at all, but I had to remove the cylinder to check the condition of the piston and piston rings. With no camshaft chain or valve parts, the 2-stroke can be easily disassembled from the waist up.
Piston rings are one of the most important parts of the engine. They prevent the combustion pressure from blowing out, prevent the engine oil from entering the combustion chamber, and transfer the high-temperature heat generated by the combustion to the cylinder.
The size of the gap is an important factor for the piston ring, which has a gap like a letter "C" before it is inserted into the cylinder, and when it is inserted into the cylinder, the gap narrows and becomes a perfect circle. If you want to completely seal compression leaks, it may seem perfect to eliminate the gap and make the rings circular, but such a shape will not fit in the piston ring grooves in the first place. If you are a Sunday mechanic who has installed and removed piston rings before, you will know that piston rings can fit in the ring groove because they are cut.
Since metal expands when heated, the friction with the cylinder increases when the closed ring expands. Therefore, an appropriate gap is set in advance so that the two ends of the "C" do not collide with each other even when the ring expands due to heating.
A four-stroke engine generally uses three piston rings, while a two-stroke engine uses two rings, which gradually wear as they rub against the cylinder during the stroke. In addition to ring wear, there is also wear that causes the cylinder bore to expand, but in either case, the gap between the piston rings and the cylinder widens slightly.
This will cause the combustion pressure to escape through the enlarged gap to the crankcase side, resulting in increased blow-by gas, and at the same time, compression will escape, reducing the force to push the piston down, resulting in a loss of engine power. For this reason, the service manual lists the standard and allowable gaps between the piston rings. For example, the service manual for the Kawasaki Z1 states that the standard mating gap is 0.2 to 0.4 mm, with a limit of 0.7 mm. For the 2-stroke Yamaha YB-1, the standard value is 0.15 to 0.30 mm, with a limit of 0.7 mm.
- Point 1: Piston rings have a proper mating gap, and must be replaced if the gap extends beyond the limit of use.
- Point 2: The standard value of the gap and the use limit follow the value of the service manual of each model.
Compression pressure blows out when piston ring tension is reduced
To make it easier to see in the image, the gap is measured at the bottom of the cylinder, but it should be done higher up. When measured with a stickiness gauge, 0.35 mm fits, but 0.40 mm does not. There is still clearance up to 0.7mm, the limit of use, but it is more worn than the standard gap limit of 0.30mm.
As well as the widening of the gap, the blowout marks on the side of the piston are remarkable. There is no noticeable damage on the cylinder or the piston, but the reason for this much pullout is usually due to the low tension of the piston rings. When I observed the piston, I found that the top ring was almost in the ring groove, so it was unlikely that the top ring was under tension even if it was in the cylinder.
Along with the mating gap, tension is another important factor. The rings that fit into the piston ring grooves are generally protruding from the ring grooves, and are inserted into the cylinders as they are squeezed together during engine assembly. The tension that pushes the ring itself against the cylinder wall is essential to minimize leakage of compression pressure.
The sustainability of piston ring tension depends in part on the ring material. Today's piston rings are made of steel, but cast iron rings were used in old and out-of-print models before the 1960s, and even in the 1970s for smaller motorcycles. Cast iron rings are inferior to steel rings in terms of strength, elasticity, and heat distortion, but this is inevitable given the technology of the time. Conversely, the steel ring was developed to solve the weakness of the cast ring.
Of course, even if the ring is made of steel, its tension will decrease as it is pressed into the cylinder and exposed to combustion heat. The service manual for the Kawasaki Z1 includes data on the gap (free space) between the top ring and the second ring. In other words, the gap narrows as the tension decreases with age.
Tension drop is a problem that should not be overlooked. When I had an oil spill on my previous Kawasaki Z1000J, the gap was within the default value, but it was caused by a tension drop. When I restored the piston because there was no problem with the gap measurement, I found that there was no tension in the piston, so I ordered a new one and compared it with the old one. The free gap was totally different, and I found that the cause of the oil rise was the decrease in the ability of the cylinder to scrape off the oil due to the tension decrease.
In addition to the tension, the ring was pressed against the cylinder by compression pressure. Maintaining compressive pressure was therefore very important.
- Point 1: Even if the mating gap is within the limit of use, the compression pressure may blow out due to reduced tension of the piston ring.
- Point 2: Cast rings used for old and out-of-print models are more prone to tension loss over time compared to steel rings.
The 2-stroke with no rotating ring is easy to see the blowout from the mouth.
The pistons are no longer available, but the 50cc piston ring set is available as an OEM part. If you can't use the piston or cylinder due to seizure, you can search for a manufacturer that sells external piston kits (TKRJ is famous for piston kits for small motorcycles).
There is no big damage to the piston itself, so I was able to restore the engine by cleaning and assembling new piston rings. Family motorcycles are often ridden more quietly than sportbikes, so there seems to be less trouble. However, it is inevitable that the tension of the piston rings will decrease with age.
The main difference between the 2-stroke and 4-stroke piston rings is that the 4-stroke rings can rotate freely along the ring groove, while the 2-stroke rings are positioned by a knock pin at the mating port. Since the 2-stroke has exhaust and scavenging port openings on the inside surface of the cylinder, the ring is designed so that the mating port is located in an area where there is no port, since the ring would be damaged if it rotated freely and the mating port matched the port location.
The fact that the ring mating position is fixed is a symptom that can be clearly seen when a blow-by occurs. When I removed the cylinder of a Yamaha Chappy, a family motorcycle that had probably never had its piston rings replaced since it was manufactured in the 1970s, and checked the piston, I found blow-by gas all the way down to the second ring.
Since there are no mechanical problems such as seizure by observing the piston and cylinder, and the mating gap has not reached the replacement limit, we believe that the cause is tension loss due to aging. The top ring that sits in the ring groove has barely expanded, and the carbon deposits all the way around the bottom of the top ring are another factor that can be determined to be low tension.
Fortunately, the piston ring set for this motorcycle was still available as a genuine part, even though it was manufactured over 40 years ago, so I was able to replace the rings with new ones, and even though I only replaced the rings, the exhaust sound from the exhaust system became more crisp and powerful. It is a pity that not all old and out-of-print models can afford new piston rings, but if you check the pistons and it is obvious that blow-by gas is escaping, replacing the piston rings should restore the engine condition.
- Point 1: It is easy to detect a loss of tension in the piston ring of a 2-stroke engine whose mating position is fixed by a knock pin.
- Point 2: Engine condition can be improved by simply replacing the piston rings that have lost tension.
See YAMAHA Moto Index Page
See KAWASAKI Moto Index Page
See Accessories for KAWASAKI Z1 (900 Super Four)
See Accessories for KAWASAKI Z1000J (KZ1000J)
See Accessories for YAMAHA CHAPPY
