Are Your Pistons Round?


The pistons in your engine are not particularly round. Well, they’re mostly round, but some parts are not and that’s important. There’s a good reason they’re like that. A piston’s operating environment strongly influences its overall shape and that form becomes complex when the piston is asked to perform multiple tasks while enduring a high order of abuse in performance applications. The piston’s job is to capture cylinder pressure via ring sealing, so the connecting rod and crankshaft can convert that pressure to rotary motion and torque.
The upper portion of most pistons is in fact round because that is the part that supports the ring pack and round rings must match up to round cylinder bores to achieve proper sealing. Everything above the bottom ring land is in fact round. Below the ring pack however, modern pistons assume different shapes whose subtleties are not always clear to the naked eye.  Piston skirts are necessary to stabilize the piston in the bore and promote the best possible ring seal by preventing the piston from rocking too much.

Pistons react to factors conspiring to destroy them or otherwise render them less effective at their intended purpose; said factors being heat, pressure, friction, lubrication and the inertial loads constantly striving to tear them apart. So, designers build-in features to accommodate these factors and help the piston successfully perform its duties. Combustion pressure results in significant side loading through the skirt which tries to deform the piston and cock it in the bore. Without skirts the ring pack would essentially represent a single point of contact allowing the piston freedom to rock and compromise critical ring function under both compression and combustion events. The skirt effectively creates two areas of contact that improve ring seal and absorb thrust loading while stabilizing piston motion within the cylinder bore.  

In performance circles piston skirts are typically recognized as either full skirt (full round) or slipper skirted. Symmetrical or asymmetrical skirt profiles further characterize slipper skirt designs with attending ovality and barrel profiles specific to any given design. Full skirting seemed logical in the early days. It created a robust piston that maintained its designed shape and held up well to extended abuse. Full skirt pistons typically utilized outboard pin bosses, longer wrist pins and had considerable mass. Full skirting is useful where combustion pressures and thrust loading are extremely high as in a diesel engine. It helps to maintain piston shape and stabilizes the ring pack under these conditions. Full round skirts can last longer due to less localized wear and a larger skirt contact area. Slipper skirt designs evolved as engineers sought to lighten reciprocating components to improve performance and adapt piston designs to accommodate stroker engine combinations and the elevated engine speeds of many modern performance engines. While skirts are necessary to ensure piston stability and alignment, redefining the piston shape below the ring pack leads to numerous advantages some of which include:

  • Reduced piston weight which safely supports higher engine speeds
  • Minimized contact patch reduces friction while maintaining stability
  • Pistons can be brought closer to the crankshaft without counterweight interference

Evolving piston technology found designers employing substantial refinements to significantly improve performance; chief among them, ovality and barrel face contouring. Both have been a staple of piston design for well over half a century.

If the concept of ovality doesn’t make your head spin, barrel contouring might. While ovality directs skirt shape on the horizontal axis, barrel addresses skirt shape on the vertical axis and helps support the single point of skirt contact required for maximum skirt efficiency. If the load bearing area has width, it must also incorporate height which defines the exact contact patch the engineer deems appropriate for the application. Again, this can be calculated and simulated, but it must be validated by actual testing. To visualize barrel, imagine a side view of the piston skirt and select the widest part of the piston where piston-to-wall clearance is measure. In many cases this is approximately one inch down from the bottom ring groove.

From this point, the piston skirt also assumes a curved contour as it follows a defined arc to the bottom of the lower ring groove where the piston is in fact smaller in diameter. Recall that the ring belt and the top of the piston are always round and smaller in diameter because it requires more room for expansion due to high heat in that area. Below that point, barrel works in conjunction with ovality to define a multidimensional point of contact and load bearing area best suited to the final application. To some degree you can think of barrel as a type of vertical ovality as it is intended to minimize friction by controlling the contact patch.

Barrel and ovality are the primary piston contours that establish piston-to-wall clearance in the cylinder. They work in conjunction with skirt size and thickness to establish the desired clearance point on the piston.

The illustration above shows an exaggerated view of the barrel shape manufacturers apply to their pistons. The widest part is the point where you check piston-to-wall clearance. That corresponds to the lower arrow on the piston below.
The upper arrow points to the area below the oil ring groove where the piston tapers in and clearance increases. In practice, you can't really tell unless you measure the piston at those two points.
 It may be useful to understand this compound shape by imagining a small balloon being inflated in the cylinder. Contact with the walls changes with inflation pressure but it can only change so much horizontally before the entire surface of the balloon contacts the cylinder wall horizontally. However, the balloon can still expand vertically and the arc to the contact point will change and lengthen the contact patch. In a sense that is barrel because the shape of the balloon always arcs away from the cylinder wall at some point. In practice, the barrel is slightly more pronounced at the top of the skirt near the oil ring groove and less at the bottom of the skirt; the difference being on the order of .002-.004-inch.

Designers have long known that full skirt contact with the cylinder walls was not only unnecessary, but detrimental to performance in the form of power robbing friction. Piston ovality or egg shape if you will, makes the piston body narrower along the minor axis and broader along the major axis (thrust surface). The widest part of the piston is in the load bearing area of the skirt. That’s why we measure piston-to-wall clearance at the center of the skirt and toward the bottom of it. Ovality is used to address piston expansion from heat. It brings a controlled one-dimensional solution to the amount of contact the piston has with the thrust face. And it offers the strength you would expect to be missing from the removal of a full round skirt. FSR (Forged Side Relief) skirts are stronger than full round designs, but they require more fine tuning of the skirt profile to match full round wear characteristics.

The degree of ovality is prompted by the amount of thermal expansion and thrust loading engineers predict the piston skirt will encounter. The actual geometric shape varies according to application. Only a portion of the skirt contacts the cylinder wall even though it is surrounded by more skirt material. The intent is to present a carefully calculated load bearing area best suited to each application. Normally aspirated pistons require less ovality than supercharged and other artificially boosted applications due to the varying heat and loading requirements.

Pistons may incorporate single ovality or compound ovality based on application, loading, thermal characteristics, piston material and the thickness of the skirt. The surprisingly complex shape is only arrived at via computer analysis and real-world dyno testing. Instead of one consistent curve, the degree of curve may change as you approach the load bearing area. Engineers can calculate this and simulate it on computers. Ovality shapes are cam-ground on special machines, but the process was time consuming and expensive. Today, CNC machining centers can cut these shapes much faster and more accurately.

If you look carefully at the piston above you can almost make out the thrust side ovality at the bottom. The piston is rotated 90 degrees from the illustration, but it is easy to see where the ovality occurs.

Most pistons, whether full round or slipper skirted are symmetrical in skirt contour; that is, both skirts are identical in size and profile. When you examine one of these pistons it is difficult to observe any discernable difference between the minor thrust skirt and the major thrust skirt. Most pistons are made with symmetrical skirts, but a different type of piston skirt pioneered and popularized by JE Pistons offers significant performance gains by altering the skirt design on the minor thrust side of the piston. The major thrust side retains the traditional style skirt with ovality and barrel characteristics appropriate to the specific application. The minor thrust side however, has a notably smaller skirt just large enough to still provide the stabilizing contact patch, but not as broad and robust as the skirt on the major thrust side. Because it is not subject to the higher thrust loads it can be reduced in size to reduce piston weight and frictional losses. The minor thrust side may also have its own degree of ovality and barrel contour that differs from the major thrust side.

JE asymmetrical pistons use a forged-side-relief design (FSR) with major and minor thrust skirts, inboard pin bosses and a shorter and stiffer wrist pin. Over the years, the skirt designs were tweaked, sometimes significantly, as designers refined their function according to changing performance requirements. The primary focus of the asymmetrical piston design is weight and friction reduction via the FSR’s smaller, lighter skirt on the minor thrust side. A key benefit of the asymmetrical piston is the shorter and lighter piston pin. On a Chevy LS engine, the piston pins are only 2.250 inches long, reducing piston weight by up to 10 grams.

The asymmetrical profiles do not throw the piston out of balance relative to the pin position The combination of lighter pin and pin offset keeps the piston balanced. JE offsets the pin toward the major thrust side making the balance of the piston on the pin axis almost ideal.  So, the smaller skirt has no effect on the balance unless you grind on the piston yourself during balancing. This should be unnecessary since the pistons are manufactured as balanced sets. Each piston is different and will have its own unique skirt profile based on application and anticipated or verified loading. Engineers have a good handle on this, but they still test multiple skirt profiles, often in the same test engine to gain a real-world picture of how the skirt performs under stress. The engine’s stroke, rod length and maximum cylinder pressure all contribute to any given skirt requirement. Supercharged and nitrous applications also play a factor and will normally have a more robust skirt on the major thrust side.

In addition to ovality, barrel and skirt symmetry considerations the forged-side-relief configuration lends considerable strength to the skirt profile as it concentrates the greater mass of the piston toward the center and behind the major thrust surface. Piston are one of the few engine parts that incorporate multiple shapes and sizes that merge into an overall profile best suited for any application. Maximum  gains in friction reduction are typically realized via stringent engineering protocols that define skirt profiles to meet the wide variety of performance conditions encountered in motorsports. As ring packs have evolved to thinner rings, the skirt contact patch became the final frontier of frictional challenges. By combining some, or all the available skirt shapes, designers have been able to reduce friction while improving piston stability and ring function.  That translates to greater performance and component durability for all racers.

Asymmetrical pistons provide a unique way of reducing friction and piston weight by making the skirt on the minor thrust side smaller than the skirt on the major thrust side.
The major thrust side of an asymmetrical piston is a full-sized skit designed with optimum load bearing and friction characteristics.
The minor thrust side still requires a skirt for stability, but it can be smaller because that side sees less thrust loading.
The accompanying crank angle diagram shows the dramatic difference in loading on the major thrust side of the piston. The profile above the reference line indicates loading on the major thrust side. The profile below the reference line is the minor thrust side.
Illustrations courtesy of JE Pistons