Piston Primer Part 1

Wayne Scraba

26 Jan 2017

Click Here to Begin Slideshow Consider the plight of the poor piston: It's constantly under assault. It can experience pressures that regularly exceed one-half ton per square inch (!) and temperatures that easily exceed 2,000 degrees. It can also experience acceleration forces (piston speed in feet/second along with g-force) that are off the chart - particularly in a stroker combination with a short connecting rod, but that’s another story. Much of the progress in piston technology you see today has come directly from the world of motorsports. Here, the limits of components (pistons included) are always being tested. Piston technology constantly marches forward. So what’s the right piston for a pretty common street and strip car – something you can choose to drive or race on the weekend? In truth, the street and strip part of the equation tend to muddy the answers. Unlike a pro race engine where parts can be examined and replaced between race events, a street and strip machine must plug along week after week. A piston for such an application must be capable of strong, reliable and repeat performance, but it also must be capable of sustained use in stop and go driving. FORGINGS AND CASTINGS - It's no secret that pistons can be manufactured either by forging or by casting. Cast pistons are designed for quiet operation. Typically, they include a steel strut next to the wrist pin boss so that expansion can be controlled (the steel strut holds the piston so that it is "permanently expanded"). While this piston will live comfortably in a low horsepower, daily driven street application, it was never intended for racing or high performance use. The bottom line is simple (and it might offend some readers): Cast pistons have serious limitations when it comes to high performance and/or racing. Forgings are a different story. During manufacture, a forged piston begins as an aluminum billet. The billet is placed in a die and stamped into the basic form by a punch. Forged pistons have a much denser molecular structure than a casting. Because of this, heat can transfer through the piston at a much quicker rate. This also means that the piston will be noisy when the engine is cold. As the piston reaches operating temperature (this is different than engine operating temperature), it expands, creating the correct operating clearances. Different parts of the piston "see" different temperatures. Because of this temperature differential, certain parts of the piston expand at varied rates, hence the cold "rattle" that's often associated with forged pistons. THE RIGHT ALLOY FOR THE APPLICATION - When it comes to high performance pistons, there are several different theories about piston alloys. This is a major over-simplification, but pistons can be divided into two groups - those with high silicon content (typically 4032 alloy) and those with low silicon content (typically 2618 alloy). The range between high and low content can run from about 11% to as low as 0.1%. There are many reasons for this spread, but in terms of characteristics, a high silicon piston has improved scuff resistance and improved ring groove "life". In addition, a piston that is manufactured with a high percentage of silicon in the alloy will not have a high rate of expansion. On the other hand, a piston with a low silicon content will often exhibit characteristics of "toughness" that are not possible with high silicon content alloys. The truth is, pistons made with 2618 alloy prove much more ductile than those made from 4032 alloy. In order to make the low silicon piston more durable, they are sometimes heat-treated to increase hardness. Obviously, real piston operating temperatures will minimize the effect of heat treating on the dome, but it still applies to the ring lands (to some degree) and more so to the skirt. Pistons with low silicon content have a higher rate of expansion, but with new piston design along with modern machining processes, the rate of expansion in a 2618 alloy (low silicon) content piston can be controlled. How does this effect piston choice for a street-strip combination? Simple: A piston that uses high silicon content can usually be installed with tighter piston-to-wall clearance figures than one with low silicon content. This "rule" isn't cast in stone. There are some very, very big caveats attached, but we'll get into those later. PISTON SKIRTS - If you carefully measure some pistons at the skirt, you'll find that they aren't round. Instead, they're elliptical - narrower through the wrist pin centerline and wider through the thrust centerline (the thrust centerline is opposite or 90° perpendicular to the pin centerline). This isn't a mistake. The ellipse shape is called a "cam grind.” Normally, the cam grind on a piston is between 0.020-0.045-inch. That isn't much, but it is critical because piston dimensions change as it reaches operating temperature. A number of pistons also feature something called a "barrel grind" or finish. This means the skirt bulges outward below the oil ring land. If you measured a barrel ground piston, you'd find it is smallest just below the top ring land. At a specific point in the skirt, the dimensions reach their peak and then decrease. Some pistons use a slightly less complex profile. It's called "taper.” What this means is the piston diameter is smallest at the top ring land and progressively becomes larger at the bottom of the skirt. As you can well imagine, both "taper" and "barrel grinds" are critical when it comes to establishing the piston-two-wall clearance. Because of taper or barrel grind built into a piston, you have to be absolutely positive about where on the skirt the piston manufacturer specifies clearance checking. You'll find that two basic types of piston skirts are common: Full skirts and slipper skirts. The slipper skirt has a smaller contact area (with the cylinder wall), but the initial forging can be made lighter than a full skirt piston (piston lightening is another matter). In the case of a full skirt piston, you'll find that the skirt isn't actually round. Instead, the shape is such that only the faces on the thrust axis of the piston actually contact the cylinder wall. When compared to a slipper skirt, the full skirt piston will still have more contact area. Because of this, the full skirt is sometimes thought to be easier on cylinder bores. In the next segment, we’ll dig deeper in the world of specialized pistons for street and strip applications. There’s a ton of info to digest and the truth is, it will all help you to figure out what’s right for your car. After that we'll move on to deck and dome surfaces and then to wrist pin bores. In the meantime, check out the accompanying photos.

Piston Primer Part 2 — Click Here to Begin Slideshow Consider the plight of the poor piston: It's constantly under assault. It can experience pressures that regularly exceed one-half ton per square inch (!) and temperatures that easily exceed 2,000 degrees. It can also experience acceleration forces (piston speed in feet/second along with g-force) that are off the chart - particularly in a stroker combination with a short connecting rod, but that’s another story. Much of the progress in piston technology you see today has come directly from the world of motorsports. Here, the limits of components (pistons included) are always being tested. Piston technology constantly marches forward. So what’s the right piston for a pretty common street and strip car – something you can choose to drive or race on the weekend? In truth, the street and strip part of the equation tend to muddy the answers. Unlike a pro race engine where parts can be examined and replaced between race events, a street and strip machine must plug along week after week. A piston for such an application must be capable of strong, reliable and repeat performance, but it also must be capable of sustained use in stop and go driving. FORGINGS AND CASTINGS - It's no secret that pistons can be manufactured either by forging or by casting. Cast pistons are designed for quiet operation. Typically, they include a steel strut next to the wrist pin boss so that expansion can be controlled (the steel strut holds the piston so that it is "permanently expanded"). While this piston will live comfortably in a low horsepower, daily driven street application, it was never intended for racing or high performance use. The bottom line is simple (and it might offend some readers): Cast pistons have serious limitations when it comes to high performance and/or racing. Forgings are a different story. During manufacture, a forged piston begins as an aluminum billet. The billet is placed in a die and stamped into the basic form by a punch. Forged pistons have a much denser molecular structure than a casting. Because of this, heat can transfer through the piston at a much quicker rate. This also means that the piston will be noisy when the engine is cold. As the piston reaches operating temperature (this is different than engine operating temperature), it expands, creating the correct operating clearances. Different parts of the piston "see" different temperatures. Because of this temperature differential, certain parts of the piston expand at varied rates, hence the cold "rattle" that's often associated with forged pistons. THE RIGHT ALLOY FOR THE APPLICATION - When it comes to high performance pistons, there are several different theories about piston alloys. This is a major over-simplification, but pistons can be divided into two groups - those with high silicon content (typically 4032 alloy) and those with low silicon content (typically 2618 alloy). The range between high and low content can run from about 11% to as low as 0.1%. There are many reasons for this spread, but in terms of characteristics, a high silicon piston has improved scuff resistance and improved ring groove "life". In addition, a piston that is manufactured with a high percentage of silicon in the alloy will not have a high rate of expansion. On the other hand, a piston with a low silicon content will often exhibit characteristics of "toughness" that are not possible with high silicon content alloys. The truth is, pistons made with 2618 alloy prove much more ductile than those made from 4032 alloy. In order to make the low silicon piston more durable, they are sometimes heat-treated to increase hardness. Obviously, real piston operating temperatures will minimize the effect of heat treating on the dome, but it still applies to the ring lands (to some degree) and more so to the skirt. Pistons with low silicon content have a higher rate of expansion, but with new piston design along with modern machining processes, the rate of expansion in a 2618 alloy (low silicon) content piston can be controlled. How does this effect piston choice for a street-strip combination? Simple: A piston that uses high silicon content can usually be installed with tighter piston-to-wall clearance figures than one with low silicon content. This "rule" isn't cast in stone. There are some very, very big caveats attached, but we'll get into those later. PISTON SKIRTS - If you carefully measure some pistons at the skirt, you'll find that they aren't round. Instead, they're elliptical - narrower through the wrist pin centerline and wider through the thrust centerline (the thrust centerline is opposite or 90° perpendicular to the pin centerline). This isn't a mistake. The ellipse shape is called a "cam grind.” Normally, the cam grind on a piston is between 0.020-0.045-inch. That isn't much, but it is critical because piston dimensions change as it reaches operating temperature. A number of pistons also feature something called a "barrel grind" or finish. This means the skirt bulges outward below the oil ring land. If you measured a barrel ground piston, you'd find it is smallest just below the top ring land. At a specific point in the skirt, the dimensions reach their peak and then decrease. Some pistons use a slightly less complex profile. It's called "taper.” What this means is the piston diameter is smallest at the top ring land and progressively becomes larger at the bottom of the skirt. As you can well imagine, both "taper" and "barrel grinds" are critical when it comes to establishing the piston-two-wall clearance. Because of taper or barrel grind built into a piston, you have to be absolutely positive about where on the skirt the piston manufacturer specifies clearance checking. You'll find that two basic types of piston skirts are common: Full skirts and slipper skirts. The slipper skirt has a smaller contact area (with the cylinder wall), but the initial forging can be made lighter than a full skirt piston (piston lightening is another matter). In the case of a full skirt piston, you'll find that the skirt isn't actually round. Instead, the shape is such that only the faces on the thrust axis of the piston actually contact the cylinder wall. When compared to a slipper skirt, the full skirt piston will still have more contact area. Because of this, the full skirt is sometimes thought to be easier on cylinder bores. In the next segment, we’ll dig deeper in the world of specialized pistons for street and strip applications. There’s a ton of info to digest and the truth is, it will all help you to figure out what’s right for your car. After that we'll move on to deck and dome surfaces and then to wrist pin bores. In the meantime, check out the accompanying photos.

No matter what you're told, a forged piston like this CP Carrillo example has a much denser molecular structure than a cast model. No amount of "alloying," steel struts or band-aid fixes will make a cast piston as desirable for high performance use as a forging.

It's impossible to know the alloy of a piston by simply looking at it. Different alloys are better suited to different tasks when it comes to pistons. For this particular application, CP Carrillo prefers to use a 2618 alloy.

Typically, there are two types of skirts in use: Slipper skirt and full skirt. This photo depicts a "slipper" skirt, which is lighter than a full skirt.

A high quality piston such as our CP Carrillo model incorporates a "barrel grind." This means the skirt bulges outward below the oil ring land. If you measured a barrel ground piston, you'd find that it is smallest just below the top ring land. See photo of the specification sheet for the “gauge point distance” – that’s where you measure for clearance.

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