Piston rings are essential seals in the combustion chamber of the engine. When these rings wear out or are installed incorrectly it will have serious consequences on the engine.
Worn piston rings can cause excessive blow-by of exhaust gases into the crankcase, reduced compression, white exhaust smoke, increased oil consumption, and if the ring fractures it could even grenade the engine. Because of this it is essential to recognize the signs of a worn or failing piston ring before it turns into a larger problem.
What Are Piston Rings?
Piston rings are located on the cylinders of your motorcycle’s engine. Many cylinders will have three different rings. The piston rings have four different and very important jobs that help keep the engine healthy.
The top ring on the piston functions to seal the combustion chamber. This seal is incredibly important for the engine as the combustion gases created from ignition are the forces that spin the crankshaft. If these gases were to leak around the cylinder head, the motorcycle would lose power and compression.
The middle piston ring also functions as a backup seal for the combustion gases. It also functions to make sure that there is a proper oil film thickness between the cylinder and the engine block. As oil is pumped into the pistons, the piston rings control the oil film thickness. This oil film is essential in reducing friction and heat transfer.
Some cylinders will only have two piston rings whereas most will have three. The third ring is primarily for the oil film thickness and is sometimes called the oil scraper.
The piston rings allow for heat to dissipate into the engine block. Motorcycle engines get very hot during normal operation so the piston rings prevent overheating by transferring the heat from combustion to the cylinder head. The piston rings contact both the cylinder and the engine block. This allows for heat transfer via conduction and heat transfer to the engine oil.
The last function that the piston rings have is to prevent knocking. When the piston makes contact with the engine block it is called knocking. The rings support and center the piston to provide smooth up and down motion for the engine’s operation.
How Do You know If Your Piston Rings Are Bad?
There are many signs that point to a bad piston ring. The first is blue smoke coming from the motorcycle’s exhaust. Blue exhaust is a clear sign of an engine that is burning oil. This is caused by an oil leak into the combustion chamber, and the cause of leak could be worn or damaged piston rings. Burning oil is a serious issue that should be addressed right away. More oil can be added to buy time, but that is not a long term fix.
The next issue you might notice with your motorcycle is poor acceleration and performance. The piston rings perform a critical function of sealing the combustion chamber. Without proper sealing, some of the combustion gases will “blow-by” the cylinder and into the crankcase. Since the gases are escaping around the cylinder and not pushing the cylinder down, the engine will have reduced performance. These leaks can cause the engine have a low compression.
Excessive oil consumption is another issue that can be caused by worn piston rings. This excessive oil consumption can occur without blue or white exhaust smoke. In summary the symptoms of bad piston rings are the following:
- Blue exhaust smoke caused by oil burning
- Excessive oil consumption
- Low compression
- Excess blow-by into the crankcase
Bad Valve Seals or Piston Rings?
Because there are multiple problems that can cause the engine to burn oil, there can be some confusion as to what to whether the engine has bad rings or bad valve seals. Thankfully there is a way to tell which is the cause of the problem without tearing the engine apart.
Bad valve seals generally cause an intermittent oil burning. You should only see white or blue exhaust from your motorcycle at start up and shortly after. However, bad piston rings will burn oil consistently throughout all operation. So if the engine is expelling blue or white exhaust fumes, it should stay consistent throughout all engine operation.
Why Do Piston Rings Fail
The most common reason for piston rings to fail is due to an engine overheating. The overheating engine can warp the cylinder head or rings. This makes it impossible for the piston ring to seal leading the problems discussed above. The cause for overheating could be due to a lack of oil which would create excess friction and heat, or it could be caused by a coolant system failure.
Other reasons for a piston ring failure could be contamination from the gas tank. The oil system could also carry contaminates capable of damaging the rings. There are many reasons why piston rings can fail, but proper maintenance like changing the oil at the specified intervals and using a quality oil can lower the risk of piston ring failures.
Two Stroke Engine Piston Rings
While most four stroke engines use three piston rings, two stroke engines generally use either one or two piston rings. Most racing two stroke engines use one piston ring.
This is because one piston ring has less friction than two allowing for slightly more power. The second reason is that there is slightly less weight. The downside to this is that cylinders with only one piston ring wear out faster due to less heat transfer.
Two piston ring seals are better at transferring heat from combustion. The added ring also provides better durability and longevity incase the sealing capability of the first ring is compromised. The down side is that the added ring brings increased friction and weight. For more information on the subject, check out Wiseco’s article on one versus two piston rings here.
Piston ring repairs require an extensive amount of work. The engine will have to be removed from the motorcycle frame and disassembled. The pistons have to be completely removed from the engine so that the old rings can be removed. Installing the piston rings does require a piston ring compressor tool.
The best way to fix piston rings is to prevent needing to replace them in the first place. This can be done by keeping up on oil and filter change maintenance, fuel filter changes, and keeping a close eye on your motorcycle’s engine temperature so that it does not overheat. If your engine does experience a piston ring failure, make sure you fix it as soon as possible as riding with damaged rings will lead to more severe problems if left unaddressed