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Pistons & Rings: Details every engine builder should know
By Larry Carley
Though pistons can often be salvaged when remanufacturing an engine, sometimes they have to be replaced. The original pistons may have worn or damaged ring grooves, wrist pins or skirts, or they may have cracks. New pistons may also be necessary if the cylinders have too much taper wear and the block has to be bored to oversize. Or, maybe you just don't want to reuse the original pistons if you're building a performance engine or a high load engine like a diesel.
Pistons can fail any number of ways. The worst kind of failure is a catastrophic explosion that spews shrapnel inside the engine. A cast piston can shatter like a grenade if it hits a valve at high rpm, or a solid object is sucked into the combustion chamber. The underlying cause may have been a broken timing chain or belt, the head of a valve snapping off because of fatigue failure (which can be caused by a lack of concentricity between the valve guide and seat), or a valve keeper breaking off the top of the valve stem or pulling through the spring retainer. In a performance engine, a close encounter between a valve and piston may occur if the engine overrevs causing the valves to float, or a valve spring breaks.
Burned and broken pistons are another problem you'll often discover when the engine is torn down. The underlying cause here is often engine overheating, detonation (spark knock) and/or preignition. This includes problems that may be lurking in the vehicle's cooling system, fuel delivery system, ignition system and emission controls.
Detonation can be caused by low octane fuel, over advanced spark timing, a defective or plugged EGR valve, a lean fuel mixture (dirty fuel injectors or a weak fuel pump), a buildup of carbon deposits in the combustion chamber, and engine overheating. Detonation can occur when fuel ignites spontaneously from excessive heat and pressure. The sudden increase in pressure can really hammer the pistons and rings causing them to break. Detonation can also damage the rod bearings and head gasket, too.
Preignition occurs when a hot spot inside the combustion chamber ignites the fuel before the spark plug fires. This also causes elevated pressures and temperatures that can sometimes burn a hole right through the top of a piston! Preignition is most often due to engine overheating and lean fuel mixtures, but can also result if the wrong heat range spark plugs (too hot) are used.
Replacing detonation- or preignition-damaged pistons in a customer's engine may temporarily return the engine to service. But the new pistons will likely suffer the same fate as the old ones unless the conditions that were causing the problems are diagnosed and corrected.
Scuffing is another condition that can damage pistons. Scuffing is often the result of overheating, but loss of lubrication, detonation and preignition can also be contributing factors. When an engine runs hot, the pistons swell. This reduces the clearance between the piston and cylinder walls. The cylinder bore can also distort adding to the problem. If the piston scuffs, it will wipe metal off the side of the piston.
Where the scuffing occurs will give you a clue as to what might have caused it. When overheating is involved, the scuffing will be primarily on the upper ring lands and on the sides near the wrist pins. There may also be oil carbon and lacquer burned onto the underside of the piston indicating it got too hot. Scuff marks on the lower skirt area often indicates a lack of lubrication (check the oil pump and pickup screen). Scuff marks on the edges or corners of the thrust sides of the piston may be the result of bore distortion. Scuffing on both thrust sides would indicate binding in the wrist pin.
Normal wear also takes its toll on pistons. The constant pressure and reciprocating motion in the cylinder bore causes wear on the piston skirt as well as the wrist pin bosses and ring lands. Elevated temperatures and high loads cause microwelding between the rings and lands, resulting in rapid land wear.
Sometimes a wrist pin will work loose and chew into the cylinder with each stroke of the piston. The underlying cause here may have been improper installation of the retaining lock rings on a full floating wrist pin, improper fit or installation of a pressed-in wrist pin, a twisted or bent connecting rod, excessive thrust end play in the crankshaft or taper wear or misalignment in the crankshaft rod journal.
Piston slap is a classic symptom of too much clearance between the pistons and cylinder bores. Piston slap is most audible when a cold engine is first started because clearances are greatest then. This doesn't necessarily mean the pistons are worn because some new engines will slap a bit when first started. But if the slap doesn't go away as the engine warms up, it usually means the pistons and/or cylinders are worn. A compression test and/or leakdown test can be used to confirm the diagnosis.
A cracked piston or one with a hole in it will usually signal its presence with a plume of blue smoke in the exhaust. There will also be excessive blowby into the crankcase, which will be evident if the PCV valve is temporarily removed while the engine is idling. A power balance test, compression check or leakdown test can be used to isolate the problem to a specific cylinder.
Any pistons that are worn or damaged must be replaced. In most cases, if one piston is bad, the others are too so all would be replaced as a complete set. Many pistons that appear to be in good condition and show no signs of scuffing may still have to be replaced because the upper ring lands are worn. If a piston with worn ring lands is reused, the rings won't seal properly and the engine will use oil. Wear or looseness in the wrist pin area would also call for replacement.
Another reason to replace pistons would be to change the stock compression ratio (as when using different heads, aftermarket heads, or when building a performance motor), or to increase engine durability. Stock cast pistons are fine for everyday driving but may lack the strength to handle higher than stock horsepower in a modified engine. Upgrading to hypereutectic pistons or forged pistons in such a case would probably be necessary.
The safest bet is to replace pistons with ones that are the same material as the original, or better. Many late model engines today are factory-equipped with hypereutectic pistons. Switching to cheaper cast pistons may be asking for trouble.
Hypereutectic pistons have a higher silicon content (typically 16 to 20 percent versus 8 to 11 percent for a standard piston), which makes them harder, more wear resistant and much stronger than ordinary cast alloys. Consequently, hypereutectic pistons are better able to resist ring pound out and scuffing. They also expand less than standard cast alloys, which allows them to be assembled with closer tolerances (helps reduce noise)
For high performance, severe service or heavy-duty applications, hypereutectic or forged pistons are usually required. Forged pistons may contain almost no silicon up to 11.5 percent, depending on the alloy and application. The important difference here is the way forged pistons are made: they are forged under high pressure rather than cast. The forging process increases the density of the metal and significantly improves its strength (up to 40 percent or more over conventional cast pistons). Forging also increases cracking resistance, and may allow a piston to survive a close encounter with a valve without shattering. Forged pistons generally run 18 to 20 percent cooler than cast pistons, too, which is a plus in high heat or performance applications. Because of these differences, forged pistons are usually the preferred choice for turbocharged, supercharged engines, and those running nitrous oxide.
Some people think that the same thermal characteristics that allow forged pistons to run cooler also causes them to swell more as they heat up. Consequently, there's a common misconception that forged pistons always require greater skirt-to-wall clearances - a notion that is not necessarily true because clearances depend on the type of alloy that is used in a forged piston, the design of the piston itself the application in which the piston will be used. Some forged alloys actually have a lower coefficient of thermal expansion than the alloys commonly used in conventional cast pistons!
One way to control thermal expansion in a piston is to manufacture it so the piston is slightly elliptic rather than round. The diameter of most pistons (forged as well as cast) measures anywhere from .010" to .035" shorter across the wrist pin axis than the diameter perpendicular to the pin (the "major" axis). This compensates for the greater mass in the wrist pin area which causes the piston to swell sideways as it heats up. This allows the piston to fill the hole as it heats up for a tighter all-round seal.
Piston growth is also influenced by the temperature differential between the top and bottom of the piston. The top can be 300° F. or more hotter than the bottom. Since the top runs hotter and swells more than the bottom, growth can be controlled by making the skirt profile taper in towards the top. The typical piston is widest at the bottom of the skirt and narrowest at the top (which is why it is so important to always measure a piston at the location specified by the piston manufacturer, which may be either perpendicular to the pin at the pin centerline, one inch up from the bottom of the skirt or at the top of the skirt).
When all these factors are taken into consideration, there can be considerable differences in recommended minimum skirt clearances between various brands of forged pistons. In some applications (such as a low compression, moderate horsepower output engine), a forged piston may be installed with the same clearances as an OE cast piston. In other applications (high compression, high power output), the pistons may need additional clearance.
Whether the piston has a coated skirt (and what kind of coating) will also affect the recommended clearance. Coatings are applied to prevent dry starts and scuffing, and to protect the piston and cylinders in case the engine loses lubrication. Most engines today have very tight piston-to-wall clearances (.001" or less) to minimize blowby and reduce piston rock. With a coating, the clearance may be reduced even more, and some coated pistons may actually be installed with zero clearance!
Replacement rings come in various types, styles and sizes. Standard size rings are okay if the cylinders are not worn excessively (which requires measuring taper with a cylinder bore gauge). But oversized rings are obviously required if the cylinders are worn and are bored to oversize.
Ring size will also depend on the pistons used (shallow groove or deep groove as well as groove height). Most late model engines have "low tension" piston rings that are thinner and narrower to reduce internal friction. Some are as small as 1.0 mm but the most common sizes are 1.2 to 1.5 mm. Rings designed for standard grooves must not be used in shallow groove pistons, nor should narrow rings be used in deep groove pistons.
The ring material as well as the facing (chrome or moly) should match the original application or be a suitable substitute for the original rings. Steel rings are used in many high output, turbocharged and supercharged engines, as well as diesels. Ductile iron rings may also be used, as these are also stronger than plain cast iron rings. Cast iron rings are still a popular choice for many older engines as well as "economy" rebuilds that are suitable for light duty, everyday driving. But cast rings cannot provide the durability or longevity of moly or chrome rings.
The top ring is the primary compression control ring because it seals the combustion chamber and takes the brunt of the heat. That's why the top ring on most late model engines is faced with a molybdenum (moly) coating. Many top rings are also steel or ductile iron, and on many Japanese engines the top ring is nitride coated 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 8.0 mm. Today that distance has decreased to only 3.0 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 run at close to 600° F, while the second ring is seeing temperatures of 300° 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 rings 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.
Rings are sometimes damaged by improper installation. Always use a ring expander to mount the rings on the pistons. This will minimize the risk of breaking or twisting the rings. which can happen if the rings are hand-installed on the pistons. A ring compressor will be needed to install the pistons in the block.
Cylinder bores must also be properly resurfaced (plateau finish is best). For plain cast iron or chrome rings in a stock, street performance or dirt track motor, hone with #220 grit silicon carbide stones (or #280 to #400 diamond stones) to within .0005" of final size. Then finish the bores with a few strokes using an abrasive nylon bristle plateau honing tool, cork stones or a flexible abrasive brush.
For moly faced rings in a street performance, drag or circle track motor, hone with a conventional #280 grit silicon carbide vitrified abrasive, then finish by briefly honing to final size with a #400 grit vitrified stone or #600 grit diamond stone (or higher), plateau honing tool, cork stones or a brush.
For stock and street performance engines with moly rings, an average surface finish of 15 to 20 Ra is typically recommended. for higher classes of racing, you can go a little smoother provided you don't glaze the cylinders.
For moly or nitrided rings in a performance motor, hone with #320 or #400 vitrified stones, and finish with #600 stones, cork stones, a plateau honing tool or brush.
Cylinders must also be cleaned to remove all traces of honing residue, and lubricated with oil before the pistons are installed. The finish and crosshatch in the cylinder bores must match the requirements of the rings that are used. Ring manufacturers typically recommend a crosshatch angle of 22° to 32° as measured from horizontal and uniform in both directions.
Ring end gaps must be checked to make sure they are within specifications. End gap is checked by placing a ring about an inch down in the cylinder bore and measuring the gap between the ends with a feeler gauge. The gap can be increased if needed by filing the ends of the ring.
To improve ring sealing, some late model engines such as Ford 4.6L and Corvette LS1 are now using a wider end gap on the second ring. The end gap on the second ring is 1.5 to two times that of the top ring. The actual specification may range from .006" to .013" greater than the top ring depending on the application. The idea here is to treat the rings as a dynamic rather than static assembly.
When the combustion pressure over the top ring is greater than the pressure between the top and second ring, it forces the top ring downward and outward to seal against the piston groove and cylinder. But if pressure builds up between the two rings, it can prevent the top ring from sealing and increase blowby. One way to maintain the pressure differential is to open up the end gap of the second oil ring. A wider end gap provides an escape route for blowby gasses that get past the top ring. This prevents pressure from building up so the top ring will continue to provide maximum sealing.
On some pistons, an "accumulator groove" is machined into the piston between the top and second ring to increase the volume of space between the rings. The accumulator groove helps reduce the buildup of pressure until the blowby gases can escape through the end gap in the second ring.
For naturally aspirated engines, a top ring end gap of .004" per inch of bore diameter is often recommended for a stock or moderate performance engine. For a 4-inch bore, that translates into a top ring end gap of .016" to .018". But this will vary depending on the power output of the engine. On performance engines, the gap needs to be increased to accommodate greater thermal expansion due to higher heat loads. An oval track motor might require a top ring end gap of .018" to .020", while a turbocharged or supercharged racing engine might need as much as .024" to .026" with a four-inch bore.
The recommended end gaps for 2nd compression rings would also the same as the top rings, with slightly larger gaps if you want to minimize pressure buildup between the rings.
The recommended ring end gap for most oil rings (except the new super narrow one-piece rings) regardless of engine application is typically .015".
GAS PORTS & GAPLESS RINGS
Another trick to improve ring sealing at high rpm is to run pistons that have gas ports behind the top ring. Combustion pressure blows through the port to help seal the ring from behind and underneath. Some use vertical gas ports with holes drilled from the top of the piston to the top ring groove just behind the ring. Others use lateral gas ports that are drilled through the bottom side of the top land and extend to the back wall of the ring groove. Gas ports work best at high rpm (above 7,000 rpm) and are not recommended for street engines.
Getting rid of the ring end gap altogether can also improve sealing, cooling and horsepower. Gapless rings eliminate the gap between the ends of the ring by overlapping slightly. Some engine builders who have switched to "gapless" top or 2nd compression rings say they've gained three to five percent more horsepower with no other changes. Gapless rings are said to allow less than 1 cubic feet per minute (cfm) of blowby and on alcohol-fueled engines, a gapless top ring or 2nd ring helps keep alcohol out of the crankcase.
Gapless rings made of hybrid iron and different grades of steel are available in most popular sizes. The rings are also offered with various wear-resistant face and side coatings. One such coating that has proven to be extremely durable is a plasma vapor deposited chrome nitride coating. The thin film coating adds hardness and wear resistance that extends both ring and cylinder life, according to one ring supplier who uses this type of coating.
By Larry Carley
You've built an engine exactly the same as the last engine, but the power seems to be down 10 to 20 horsepower on the dyno. Could it be excessive blowby because of a ring sealing problem? One way to find out is to measure crankcase blowby.
A blowby flow meter can tell you precisely how much blowby is occurring inside the engine. Unlike a cranking compression test or a static leakdown test, a blowby test actually measures the volume of gases that are entering the crankcase past the piston rings. The flow meter allows you to measure blowby from any engine speed, all the way from idle to wide open throttle.
A blowby test requires a blowby flow meter. The meter measures airflow, and is attached to either the crankcase vent on a valve cover breather, or the PCV valve fitting. On a V6 or V8 engine, the opening on the opposite valve cover must be temporarily blocked so all the airflow from the crankcase will flow past the meter.
When the engine is running, all blowby that leaks past the rings will flow through the crankcase, out the valve cover opening and through the blowby flow meter sensor. The meter outputs an analog voltage signal that ranges from zero to five volts. The display can then be converted into units that show you the volume of airflow per unit of time. Most engine builders typically display the reading in cubic feet per minute (cfm), though heavy-duty engine builders more often use cubic feet per hour (cfh).
One supplier of blowby flow meters said contrary to what many people think an engine typically has more blowby at idle than at higher rpms. As the speed goes up, the rings actually seal better and blowby drops.
How much blowby is normal? Dividing an engine's maximum horsepower output by 50 will give you a ballpark number for how much blowby you would normally expect to see. For example, a street performance engine that makes around 500 horsepower will typically have about 10 cfm of blowby with conventional pistons rings and ring end gap tolerances. Higher performance engines that are built to tighter tolerances will usually have less blowby, as might those with gapless piston rings. An 800 to 900 horsepower NASCAR motor, for example, might only have 5 cfm of blowby.
Less blowby means more usable horsepower. Being able to baseline the actual blowby in an engine means you can then go back and try different ring configurations, ring types (conventional or gapless), different ring end gap settings and cylinder wall finishes to see which combination gives the best seal and the least amount of blowby.
Measuring blowby has been one of the best kept secrets with performance engine builders because it allows them to see how well the rings are or are not sealing. It also allows them to detect any ring flutter that may be occurring within a particular rpm range, and to then change the mass or end gaps of the rings to minimize the problem.