How Piston Rings Work
An engine is a simple mechanical device, an air pump really, that uses pistons and valves to pull air in and push air out. The pistons in an engine block must be sealed inside each cylinder in order for the process to be efficient. Because the pistons travel through the stroke millions of times throughout the life of the engine, the seals are critical. This is done with a series of rings that go around the top half of the piston, sealing it to the cylinder wall. When the piston rings wear out, the engine’s efficiency suffers.
How they work
Piston rings are made of metal, typical stock engines use cast iron rings, while high-performance engines may use ductile iron rings with a chromoly or chrome face. Cast iron is good for stock rebuilds, but that is about it.
Ductile iron rings are economical and good for performance engine builds. When left as is (no additional material facing), they are generally good to about two horsepower per cubic inch, so a naturally-aspirated 600-hp 350 cubic inch engine could safely run plain ductile iron rings. Stepping up to more power or adding boost or nitrous, the rings would need to be upgraded.
Many performance piston rings use the ductile iron ring and add a facing, such as plasma moly, which adds hardness and protection for added RPM and boost. Many top-fuel dragsters run plasma moly ductile rings for their durability, but they also get changed every run, so take that for what it is worth.
Modern engines have increasingly narrower piston rings, this is in part because of better manufacturing processes and material refinements. The narrower the ring, the more fragile they become. For this reason, a performance build using narrow rings means that you need the strongest material. Steel-based rings are 20-percent (or more) stronger than ductile iron rings, making them the ideal choice for narrow rings.
Horsepower is not the only benefit of a narrow steel ring. Because the ring is so narrow, it reduces friction and can conform to the cylinder wall better at the same time, so the engine is more efficient and seals better. This means less oil blowby and less emissions. Steel rings last longer, but they do take longer to break in. Steel rings may be plain carbon steel or stainless steel, however stainless rings cannot be used in a standard engine bore without titanium or chromium nitriding.
Ring coatings, such as plasma moly, titanium, chromium, and ceramic are good options for performance engines. These coatings are often used for specific engine builds, such as dirt track (chromium for dirt resistance). Chrome nitriding does not flake off like a typical chrome plating.
Moly rings break in faster than chrome, and they have better scuff heat resistance. The drawback for moly-faced rings is that under detonation, the moly material can be damaged significantly. Moly rings are not good for alcohol fuel either.
The design of the ring profile is another variable. Most stock rings use a square-face for both the top and second rings. This design is functional and seals the cylinder, but there are better alternatives. Barrel-shaped profiles, either center or offset, provide excellent sealing and long life. A square-faced top ring will eventually wear down to a natural barrel shape, having this shape from the beginning provides longer ring life.
The second ring is usually square, tapered or napier shaped. A tapered face is just that, a slightly ramped edge. This is for improved scraping of the cylinder walls to remove oil. The main job of the second ring is oil control, sealing the combustion chamber is a distant second. The Napier profile is a square-faced ring with a hook groove carved into the underside. This groove pulls oil away from the cylinder wall, reducing friction and improving oil control. Napier rings are susceptible to damage from heavy boost. This is due to the hook thinning out the ring at the edge.
The top ring is designed to control 90-percent of the combustion gasses, the second ring handles the last 10-20 percent. The last set of rings are called the oil rings, these rings scrape the majority of the oil off the cylinder walls. The oil rings are actually a series of 3 pieces- 2 thin chrome-faced or nitrided steel rings with a corrugated expander ring in the center. Performance and stock engines alike use the same design and for good reason- it works. The 2 thin rings seal the cylinder walls independently, unlike a single oil ring. The corrugated expander provides a drain relief to get the oil out of the way quickly.
When Piston Rings Fail
Piston rings must deal with an assortment of issues. Bad gas (detonation and pinging), dirty air and fuel, and contaminated oil all reduce the life of piston rings. Maintaining the filters on your engine and regular oil changes make a big difference in how long the rings last. Once the rings have worn down, their ability to seal the combustion gases will become apparent.
The first sign of worn piston rings is blowby. This will usually be seen through the tailpipe. Puffs of blue smoke out the exhaust means the engine is burning oil. You may notice that the engine oil gets low faster than before. Oil smoking first appears during engine cold starts. As the engine warms up, the pistons and rings expand, sealing the walls, reducing how much oil is blowing past the rings. Eventually, the rings wear to the point that there is constant blow-by, and the car smokes all the time. This can also be due to worn valve seals as well.
Another issue with blow-by is combustion gases entering the crankcase. This means fuel and combustion byproducts in the oil. As these chemicals permeate the oil, the oil loses its viscosity and the ability to cool and lubricate the engine. You must change the oil more often to keep the engine clean.
Eventually, the wear gets so bad that there is a loss of power, too much of the combustion gasses are being lost to the crankcase and too much oil is getting into the combustion chamber. This leads to fouled spark plugs and a poorly running engine.
There are some very good liquid repair chemicals available for mildly worn engine rings, such as Mac’s Stop Smoke. These products coat the rings in a thick oil film, which will help them seal the walls of the engine for a time. They do not last forever and only work on mild to moderately worn engines. The effects of worn rings are masked for a time, but the engine will need a rebuild.
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