Piston rings are circular metal bands inserted near the piston heads. They are composed of two parts: oil control rings and compression rings. Oil control rings are mostly responsible for cleaning the cylinder walls off excess oil. Compression rings on the other hand mostly keep the seal between the combustion chamber and the rest of the cylinder, and it transfers heat from the piston to the cylinder wall. It is important to take note of some elements when replacing the compression springs. Picking the wrong set can greatly affect engine performance.
Assuming you are picking a set of identical rings that fit the engine bore, here are a few things to consider to filter your options.
Iron vs. steel
Compression springs can be made of either cast iron or steel. Here's a quick overview.
- Cast Iron: Old stock engines usually come fitted with cast iron piston rings. As the lowest grade of rings, these are suitable for low power engines that don't heat up as much as turbocharged engines. This is also the least expensive, so cast iron is perfect when on a tight budget.
- Steel: These rings can either be in chrome or molybdenum (moly). Steel rings are fit for powerful engines because of its high heat tolerance. It is also more durable and wear resistant compared to cast iron. Chrome or moly rings are usually used for dirt vehicles.
As a general rule, only fit cast iron rings on engines that require them as a minimum. Never downgrade from a higher level ring to a lower level ring.
Size and thickness
Remember to balance the size and thickness of the ring between the ring insert's gaps, and the optimum performance of the engine. Piston rings expand when heated. A larger ring is good in terms of sealing the combustion chamber to avoid blow-by. More metal also means faster heat transfer from piston to the engine. However, install a ring too large and it might cause extra friction and wear to the cylinder wall, shortening ring life.
Grooves and edges
The little grooves at the edge of the rings play a role in both engine compression and engine oil control. Different cuts also have varying effects on a piston's vertical uplift and heat transfer. There are styles like L-shaped, rectangular, keystone, taper, and faced, depending on the availability and compatibility to your engine. Pick the right style that fits your needs. A bad design can damage the seal and effectiveness of the engine.