Piston Replacement - Making Engine Masterpieces
By Larry Carley
Creating Engine Masterpieces from the Inside Out
In the Middle Ages, a piece of work such as a sword or suit of armor was presented to a medieval guild to show that its maker was worthy of the rank of master craftsman. Those objects were then referred to as masterpieces, and gave its creator a great honor among his peers.
Today, that same satisfaction is obtained by those master engine builders who create masterpiece powerplants that exemplify both engine magnificence and performance.
These engine craftsmen understand that to create a renowned product, you must be meticulous with your components. This is good advice to those of you who choose to rebuild an engine from the inside out.
Take note of the knowledge from engine specialists who have gone before you and who are ready to share with you their secret to building perfect engines it begins with the pistons.
Though pistons can often be salvaged when remanufacturing an engine, usually they have to be replaced, especially if you are building a custom engine. 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.
Master Engine builders will tell you that the secret to building perfect engines begins with the pistons.
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 or an 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 or 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 indicate 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.
When to Replace
Scuffing (top photo) is often the result of overheating. The location of where the damage occurs on a piston (bottom photo), will give you a good clue as to what might have caused the problem.
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).
HP and Severe Duty
Burned and broken pistons (bottom photo) are a problem you’ll discover when the engine is torn down. This may be due to an engine overheating, detonation (spark knock) and/or preignition.
The underlying problem of this failure (top photo) may have been
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.
Handling the Heat
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!
Next Month, we’ll take a look at a major influencer to the masterpiece engine the piston rings.