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Combustion Barriers Piston Rings are Critical to Engine Health


9/1/2003
By Larry Carley

With regular oil and filter changes, many engines now go 150,000 miles or more without developing an appetite for oil - and that's been a real challenge for engine parts suppliers and distributors.
 

Years ago, piston rings were routinely replaced along with bearings and other internal engine parts at mileages far below what's common for today's engines. With regular oil and filter changes, many engines now go 150,000 miles or more without developing an appetite for oil - and that's been a real challenge for engine parts suppliers and distributors. Far fewer engines are being rebuilt today than even a decade ago. And those that are being rebuilt are being rebuilt at much higher mileages than ever before.

The condition of the piston rings is one of the factors that determines the health of an engine. The rings form a barrier that separate the combustion chamber from the crankcase. The rings seal the pistons against the cylinder wall to prevent combustion pressure from blowing past the pistons. They also scrape oil off the cylinder wall as the piston travels down to prevent oil from being burned by the engine. So if the rings are worn or broken, the engine will have low compression, lots of blowby and will burn oil.

THREE RING CIRCUS
Almost all passenger car and light truck engines today have three rings on every piston. A few diesels have four, and some exotic racing engines use only two. But for everything else, three is the magic number. Each ring has its own special job to do, and all three work together to seal compression, minimize blowby and control oil consumption.

The top ring is the primary compression control ring because it seals the combustion chamber and takes the brunt of the heat. The top ring on most late-model engines is faced with a molybdenum (moly) coating to reduce wear and improve durability. Chrome is another facing material that may be used to improve wear resistance. Many top rings are made of steel or ductile iron, which are tougher materials than ordinary cast iron. On many Japanese engines, the top ring has a gas nitride coating to improve durability. Chrome rings are also used in many Japanese engines.

In addition to sealing combustion, the top ring also helps cool the piston by conducting heat from the piston to the engine block. On most late-model engines, the number-one ring is located very close to the top of the piston. A decade ago, the land width between the top ring groove and piston crown was typically 7.5 to 8 mm. Today that distance has decreased to only 3 to 3.5 mm in some engines. This minimizes the crevice just above the ring that traps fuel vapor and prevents it from being completely burned when the air/fuel mixture is ignited (this lowers emissions). But the top ring's location also means it is exposed to much higher operating temperatures.

The top ring on many engines today runs at close to 600 degrees F, while the second ring sees temperatures of 300 degrees F or less. Ordinary cast-iron compression rings that work great in a stock 350 Chevy V8 can't take this kind of heat. That's why many late-model engines have steel or ductile iron top rings. Steel is more durable than plain cast iron or even ductile iron, and is required for high-output, high-load applications including turbocharged and supercharged engines, as well as diesels and performance engines.

Under the top compression ring is the number-two ring, which is the second compression ring. The number-two ring assists the top ring in sealing combustion and also helps the oil ring below it with oil control. Most second rings have a tapered face with a negative twist. This creates a sharp edge that scrapes against the cylinder wall for better oil control. Some new second-ring designs are now using a "napier" style edge that has more of a squeegee effect as it scrapes along the cylinder wall. This helps reduce friction and oil consumption even more.

The third ring is the oil ring. This is typically a three-piece ring (though some are four-piece, two-piece or even one-piece) that helps spread oil on the cylinder wall for lubrication and scrapes off the excess oil to prevent oil burning. In three-piece oil rings, there are two narrow side rails and an expander that wraps around the piston. The expander exerts both a sideways and outward pressure on the side rails so they will seal tightly against the cylinder walls.

NOTHING LASTS FOREVER
The constant up and down scraping of the rings against the cylinder walls eventually wears both the rings and bore surface. The cylinders wear most at the top because that's where loads and temperatures are the highest. This produces taper wear in the upper part of the cylinder that decreases compression and increases blowby and oil consumption. Taper also causes the rings to flex in and out as the piston moves up and down, further increasing ring wear and the risk of ring failure.

Rings can also be damaged by overheating, preignition and detonation. Detonation is especially hard on rings because it can cause them to break. And once a ring breaks, that's it for compression and oil control. The engine will smoke like a mosquito fogger until the problem is fixed. A broken ring can also tear up the cylinder bore, possibly damaging it to the point where the block cannot be honed, but must be replaced or sleeved to repair the damage.

Rings also fall victim to dirt. Unfiltered air entering the engine carries with it microscopic particles that are abrasive to the rings and bearings. Over time, this can greatly accelerate ring and bearing wear. For this reason, the air filter should never be removed or replaced with a screen or low-quality filter that does a poor job of trapping dirt.

Typical symptoms of worn rings include low compression, oil burning, spark plug fouling and elevated hydrocarbon emissions. Excessive blowby into the crankcase will also shorten oil life by dumping a lot of moisture and unburned fuel into the oil. The PCV system will help suck out some of the vapors, but the rest will form acids and sludge that can further damage the engine.

Some people say buying oil is cheaper than paying for an overhaul or a new engine. But an engine with worn rings and low compression also wastes fuel and may not pass an emissions test. Worn rings can also make the engine hard to start during cold weather. So eventually the rings will have to be replaced.

RING JOB SALES OPPORTUNITIES
By the time most engines need a ring job, they also need a lot of other work, too. The valve guides and valves may also need attention. Increased bearing clearances and oil pump wear may be hurting oil pressure. The engine may be making noise, running poorly, misfiring or suffering other effects of advanced wear. At this point the vehicle's owner has to make a decision: Is the engine worth fixing, would it be cheaper to replace the engine, or should he forget it and find another vehicle?

"Repowering" an existing vehicle is often the most affordable option, provided the rest of the vehicle is in reasonably good condition. So if the vehicle's owner opts for either a ring job or a complete overhaul, you've got a potential customer.

The key point here is that the customer will need a lot more than a set of replacement rings. He'll need gaskets, oil and a new oil filter. He may also need pistons, bearings, an oil pump and other engine parts. And he'll need special tools: a ridge reamer, a rigid hone or honing brush, a ring expander, a ring compressor, feeler gauges and a torque wrench.

Doing a ring job requires pulling the cylinder head(s), dropping the oil pan and pulling the pistons out of the engine block. "Putting Humpty Dumpty back together again" requires some preparation work to the cylinder bores, replacing anything else that's worn or damaged, installing new head gaskets, pan gaskets and manifold gaskets (and head bolts, too, depending on the application), refilling the crankcase with oil and the cooling system with antifreeze. Consequently, the parts list needed to complete the job may be rather lengthy.

At the very least, you'll sell an overhaul gasket set along with the ring set. If the cylinders are worn, the block will have to be bored to oversize, which requires new oversized pistons and oversized rings.

Tearing the engine apart also means the customer may need fasteners and thread repair inserts. Old rusty bolts that haven't been disturbed for a hundred-thousand miles or more aren't going to come apart easily. If the engine is a later model one with torque-to-yield (TTY) head bolts, most vehicle manufacturers recommend replacing the bolts. TTY head bolts are designed to stretch slightly when they are tightened down. If reused, there's a good chance they may break. The risk isn't worth it, so that's why they should always be replaced with new TTY bolts.

Old hoses and belts should also be replaced at the same time, along with spark plugs and plug wires that many not have been changed for a long time. A new air filter will help keep the engine clean, and fresh coolant in the radiator will help maintain corrosion protection and an even operating temperature. Many technicians also recommend replacing the thermostat and oxygen sensor when major engine repairs are made.

Another item that may be overdue for replacement is the timing belt if the engine has a belt-driven overhead cam. The recommended replacement interval for timing belts is usually around 60,000 miles, and many belts have never been changed. A timing belt failure can be very damaging if the engine is an "interference" application. In these application, the piston can hit the valves if the timing belt breaks.

 















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