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.

Piston Primer Part 2

Click Here to Begin Slideshow

Last issue we began our research into pistons for street and strip applications. In that segment we examined forgings versus castings and various alloys along with piston skirts. This time around, we’ll look at ring lands and ring groove clearance. Although it’s easy enough to gloss over this stuff, it’s actually extremely important in a street and strip combination. Here’s the inside story.

RING LANDS - Just how important are ring lands in a piston? Extremely. The most common piston configuration makes use of three rings - compression, second and oil. Two ring combinations (compression and oil) have seen some success in drag racing, but for cars that see street use, they're the exception rather than the rule. In all applications, the ring land is not barrel or cam ground, even if the skirt has these features. Instead, all ring lands are round (there have been some exotic exceptions over the years, but they're beyond the scope of our series). The overall diameter of the ring land is smaller than that of the piston, and in many cases a small vertical taper is present (see above section on skirts). There's a reason for this size differential: As the piston reaches top dead center, it "rocks" in the bore. If the ring land is the same diameter as the skirt, then the rings would be required to handle the thrust load (which they really don't like to do). Instead, that job is left up to the skirt.

You'll often hear another term when talking about ring lands, and that's "tilt." Many piston companies claim to include it in their pistons, but not all actually have it. According to some manufacturers, the actual ring grooves should have a positive "tilt.” This machining operation tilts the ring package upwards by a very small amount (in this sort of piston, the tilt is in the area of 2 to 3 ten-thousandths of an inch). The tilt is important in that it helps the ring seating. And by the way, the tilt is included in all of the ring grooves (top, second and oil). Some pistons of questionable quality could have a negative tilt, which actually hurts performance more than it helps.

That's not the end of ring land technology, either. The location of the ring grooves is often dictated by the compression height of the piston, the size and depth of the valve notches and, as you might expect, by the overall dimensions of the ring package. Most Detroit-built engines have the top ring 0.300-0.400-inch down from the deck surface. In an endurance racing engine, the top ring is typically 0.125-0.150-inch down while heavily modified race engines (for example, a drag race Pro Stock application) can vary from 0.060-0.100-inch down from the deck (although 0.060-inch is likely cutting things pretty close). Moving the top ring closer to the piston deck offers a number of advantages, not the least of which is minimizing the "dead air" space between the top ring land and the deck. As this space is reduced (by moving the ring up), the amount of trapped combustion gasses is also reduced. In the end, the combustion process is cleaned up considerably when the rings are moved upward. Horsepower increases.

When thinking about ring location, you should also consider how much piston material is found between the rings. While the location of the second ring isn't as critical as the distance from the deck to the top ring, it is important. The second ring doesn't see temperatures anywhere close to those encountered by the top ring, but the land between the top two rings must also support the combustion pressure exerted upon the top ring. Because of this, the width of the first land area is usually between 0.150-0.180-inch in a hi-performance piston and greater in a stock application. As far as the land area between the second and third ring is concerned, many high performance and race pistons incorporate a width of between 0.070-0.125-inch. This is narrower than the first land, but keep in mind that combustion pressures aren't present and overall temperatures are much lower. This is also a good time to mention that some pistons have "stepped" lands. This simply means that the land area between the top and second rings is slightly smaller in diameter than the area between the second and oil rings.

Certain aftermarket high performance and race pistons include a groove that is milled into the land area between the top and second ring (for example some manufacturers ball mill the groove – CP Carillo uses a unique v-shaped groove). This groove is often referred to as an "accumulator groove.” The idea behind it is to reduce pressure buildup between the pairs of rings. Theory has it that the buffer groove will stop the top ring from lifting at the bottom of the groove. The groove effectively maximizes ring seal and increases engine vacuum.

Two different types of oil drain back holes are commonly used: Drilled examples and slotted grooves. In most cases, the drilled format makes for a stronger piston, but it does allow more heat to enter the skirt area. The vast majority of race pistons are constructed with drilled drain back holes. The primary reason is strength, since the slotted system makes for a more flexible skirt.

RING CLEARANCE - How important is ring clearance in the groove? It's critical. Unfortunately, some folks (and that includes plenty of racers) tend to ignore this factor. Basically, the ring grooves must be smooth - and this is especially critical for the compression ring. What makes the clearance so crucial? During the compression stroke, the ring drops to the bottom of the groove, eventually sealing against the machined surface. During the power stroke, the piston moves down in the bore. The ring then moves up in the groove and eventually seals against the top of the machined surface. As you can well imagine, a poor finish in the groove will not allow the ring to seal tightly. Because of this, the pressure will leak past the rear (or back) portion of the ring. Evidence of this leakage can be found in excessive heat discoloration or carbon buildup in the land area between the top and second rings.

One more factor that affects the ring seal is the vertical clearance of the ring. Typically, a race piston never exceeds 0.002-inch vertical clearance (production line street engines often have clearances of between 0.002-0.004-inch). More than that and the ring will leak. Less than that figure and you run the risk of seizing the ring in the groove. A ring that can't turn freely in the groove will not clean carbon out of the groove, and will not be free to expand when combustion pressures enter the groove.

Machining plays a critical role in ring performance. And some of that piston machining capability might surprise you. Over the years, CP Carrillo has concentrated heavily on new machining methods. During the process, the company used highly accurate inspection equipment to determine how machining methods affected the flatness of things such as the ring groove. This investigation ultimately led them to ring machining tolerances (flatness) that are measured to one millionth of an inch — something that had never before been available. As a result, CP Carrillo can machine the ring groove absolutely flat. Coupled with an equally sophisticated ring, engine power goes up and so does ring sealing.
If the piston has tight vertical clearances, the ring back clearance figures become even more important. Gas pressure enters the area behind the ring during combustion. This gas pressure forces the ring out against the cylinder wall. If there is too much back clearance (which in turn creates too much volume for the combustion gasses to fill), it takes too long for the pressure to build and force the ring outward. Naturally, a smaller amount of back clearance will increase the speed and the force at which the ring will exert pressure upon the cylinder wall. Typically, a production line engine piston will have as much as 0.040-0.050-inch of back clearance. In a race or high performance application, the backside clearance figure can be reduced to as little as 0.020-inch (depending upon the ring configuration and the piston design). Of course, the back clearance can't be so small that the ring protrudes past the ring land.

There's one exception when it comes to vertical clearance, and that's when gas ports are used in the piston. The bottom line with gas ports is simple. They provide a method of supplying combustion pressure directly to the backside of the piston ring. Because of that, the ring vertical clearance can be reduced, and ring flutter can be reduced significantly. In the end, ring seal goes up and so does horsepower. Unfortunately, gas ports are only suited for use with engine combinations that see frequent tear downs (and that means gas ports don't see use on engines found in street-driven musclecars). Carbon can plug the gas port holes and because of the nature of a pressurized ring, ring and cylinder wall wear is much faster than with a more conventional set up.

Typically, gas ports consist of twelve to sixteen 0.040-0.060-inch holes drilled vertically through the piston deck that intersect with the backside of the compression ring groove. Keep in mind that gas porting is most advantageous when a narrow face, lightweight piston ring (such as the 0.043-inch ring) is used.

That wraps up this issue's look at pistons. In the next segment, we’ll examine deck and dome surfaces along with compression heights. That info is critical when it comes to the operation of your engine. After that we'll conclude this series with a look at wrist pin bores and the areas and components surrounding them. Watch for it.

Piston Primer Part 2 1

This is the most common piston ring arrangement you'll find: Three rings (top, second, oil). The ring land is not barrel or cam ground -- even if the skirt has these features. The ring land is round. The overall diameter of the ring land is smaller than that of the piston, and in many cases a small vertical taper is present.

Piston Primer Part 2 2

The location of the ring grooves is important. This CP Carrillo street-strip example uses a top ring land that is .250-inch down. Moving the top ring closer to the piston deck has a number of advantages, outlined in the text. If you run nitrous, though, don't be too quick to order pistons with extremely high ring grooves.

Piston Primer Part 2 3

The second ring doesn't see temperatures anywhere near those experienced by the compression ring; however, the land between the top two rings must also support the combustion pressure exerted upon the top ring. Because of this, the width of the first land area is usually between 0.150-0.180-inch in a high performance piston and greater in a stock application.

Piston Primer Part 2 4

In this CP Carrillo piston, an accumulator groove is machined in the land between the top and second rings. The buffer groove will stop the top ring from lifting at the bottom of the groove. The groove effectively maximizes ring seal and increases engine vacuum.

Piston Primer Part 2 5

Pistons come with one of two possible styles of oil drain back holes behind the oil ring: Drilled versions or slotted grooves. The primary reason for drilled holes is strength, because slots make the skirt more flexible.

Piston Primer Part 2 6

One of the most important aspects of a performance piston is the vertical clearance of the ring. Race pistons seldom exceed 0.002-inch vertical clearance (street cars often have clearances of between 0.002-0.004-inch). More than that and the ring will leak.

Piston Primer Part 2 7

Ring back clearance is critical too. If there is too much back clearance (which in turn creates too much volume for the combustion gasses to fill), it takes too long for the pressure to build and force the ring outward. A smaller amount of back clearance will increase the speed and the force at which the ring will exert pressure upon the cylinder wall.

Back to Post
  • David Lee

    You mention stepped piston, but do not explain the reasoning or the pros or cons of doing so. The only things that spring to mind are 1) it would be a few grams lighter and 2) it would place more burden on the skirts and rings to stabilize the piston.

    I learned a LOT in this article. Never knew about “tilt:” for instance. Very interesting. But to really understand the disign choices to be made, don’t we also need a detailed discussion, including the interplay between the many (approaching myriad) designs of rings currently in vogue (including TS flexi rings with their radial interior grooves).

Copyright © 2005-2017 RacingJunk.com All Rights Reserved.

Designated trademarks and brands are the property of their respective owners. Use of this Web site constitutes acceptance of the RacingJunk.com
Terms of Use, Classifieds Disclaimer, Privacy Policy, and Cookie Policy