Full-Round vs. Strutted: Piston Forging Designs and Skirt Styles Explained

June 28, 2018 / by Kevin Bailey

While unique piston designs may seem endless, Wiseco pistons can be traced back to either a full-round or strutted forging design. Here, we dive into the theory behind piston forging design and skirt style.

Old School Cast Piston
Early pistons were cast, with very simple shapes and features.

Rewind back to a time when piston manufacturers were pouring molten metal in a casting of a seemingly simple shape, ultimately producing a tall, cylindrical looking piston. These pistons did not feature much variation in shape, as they were designed to be used in the low-power engines of the old days.

Fast forward to today, a time when you can walk down the aisles of a piston manufacturer’s plant and see countless combinations of piston skirt, crown, and overall design features. Current engine designs are very demanding on pistons, with cylinder pressures, heat, and power adders testing the limits of piston integrity.

Wiseco Forged Pistons
Piston skirt styles can vary greatly from piston to piston, but can still be categorized into either a full-round or strutted forging design.

With over 75 years of designing and manufacturing pistons, Wiseco has seen the horsepower evolution first hand. To stay ahead of the next curveball engines might throw, Wiseco piston designs might see many different features catering to the specific needs of the engine they’ll be powering. Because Wiseco pistons are forged, all the pistons start from a certain forging design. Many different pistons can share a forging design, but the design of the forging addresses the critical elements the piston needs, including skirt design, profile, and taper.

Before diving into specifics behind different forging designs, it’s important to understand the overall shape and profile of a piston, and the reasoning behind them.

Wiseco Piston Skirts ArmorGlide
Piston skirt style is a critical characteristic necessary to meet the needs of a wide variety of unique engines.

Piston Shape

Each design feature on a piston has an intended job. The piston’s job itself is to seal cylinder pressure with the help of the piston rings. Effective sealing of cylinder pressure creates compression, which allows the fuel and air mixture to combust, and create power. This is why piston ring design is equally as important as piston design, which you can read more about here.

Although not commonly visible to the naked eye, pistons themselves are designed with a certain level of ovality and profile taper, meaning they are not perfectly round, and their diameter is larger near the bottom of the skirt than at the crown.

Wiseco Piston Profile and Ovality
This diagram provides a visual representation of piston profile and ovality.

The connecting rod is forcing the piston upward, and combustion is forcing the piston downward, but neither of these forces are applied evenly to the center of the piston. Under engine operation, piston skirts encounter a major thrust load and minor thrust load, so, the force is not equally applied across the piston. With ovality in the piston design, the piston can deform during engine operation and distribute the load evenly on the skirt panel to minimize friction. Ovality and skirt design also helps keep the piston stable and supported in the bore, allowing the rings to achieve optimum seal.

Wiseco Piston Thrust Load
The major thrust side skirt of the piston experiences more load than the minor thrust side.

The profile taper of the piston addresses the difference in temperature at the piston crown and at the bottom of the skirt. The piston crown is consistently first in contact with combustion during engine operation, and this consistent higher temperature results in a higher level of material expansion than at the bottom of the piston.

We dive even deeper into piston profile and ovality in this article.

Piston Skirts

Wiseco Pistons Powersports
Without skirts, pistons would experience a great deal of rock, and not be able to sufficiently seal compression.

First of all, why were pistons even designed with skirts? It’s the crown and rings that do the pressure sealing, right? While this is true, a piston without skirts would be exposed to immense rocking back and forth during operation. Without support from the skirts, piston rings would not be able to properly seal compression due to excessive piston movement. No seal, no power. Piston rock can also be harmful to the cylinder walls and other rotating assembly components, with sporadic movement unevenly distributing the weight of the piston throughout operation.

Piston skirts create more than one area of contact with the cylinder wall, giving the piston stability. A majority of load on the piston will be on the major thrust side. While the minor thrust side experiences less load, the piston still has the same area of contact as the major—unless it’s an asymmetrical design, which we’ll discuss in a few paragraphs—minimizing piston rock and further equalizing friction and wear from cylinder wall contact.

Wiseco Piston Skirts

Forging Design

This leads us to piston forging design. Wiseco is the only piston manufacturer that forges their pistons in the U.S., and having a plethora of forgings on hand is key to the ability to manufacture all the different pistons that can be made with them. Wiseco forging designs can be generally categorized into full-round and strutted. A key characteristic of a forging is the piston skirt design, and these terms refer to the style of skirt the piston is designed with.

Wiseco Strutted Pistons
Wiseco forging designs can be categorized into full-round and strutted. Skirt styles under the strutted category show a lot of variance depending on the engine application.

A full-round piston is somewhat self-explanatory. The skirt shape carries all the way around the circumference of the bottom of the piston. The full-round (don’t forget, it’s technically not perfectly round!) design is somewhat the ‘classic’ design of modern pistons.

Some pros to the full-round design include:

  • Maintains shape very well under stress due to material symmetry all the way around the piston
  • Holds up well to extended abuse
  • Robust design allows for resistance against extreme cylinder pressure and heat, such as in diesels
  • Larger skirt contact area against the cylinder result in less localized wear
Wiseco Full-Round Piston Wiseco Full-Round Piston
Full-round pistons maintain their symmetry and are highly-resistant to high cylinder pressure, making them perfect for some engines, and not so much for others.

However, the full-round design is not right for every engine. Cons include:

  • Greater piston weight due to amount of material and longer wrist pins
  • Weight can hinder faster engine speeds, and put added stress on other rotating assembly components

We asked Dave Fussner, long-time R&D Manager at Wiseco, for his input on full-round designs: "It provides long service life, but tends to be one of the heavier forms used. It is heavier because of the form of the piston itself, and heavier because the wrist pin is longer. These pistons do tend to be longer lasting, and are kinder to the cylinder wall wear surface. These skirt designs lend themselves to older high-performance engines."

Strutted Designs

As modern performance engines evolved, more and more builders began testing with more aggressive stroker designs and elevated engine speeds. For ideal performance, full-round pistons needed to shed some weight, but maintain integrity.

The term ‘strutted’ applies to a multitude of visibly different skirt and undercrown characteristics. The first level of shedding weight and friction is a version of the strutted design that closely resembles the full-round. The key difference between the two is the bottom of the skirt is essentially “cut off” on the sides of the piston, under the wrist pin bores. This achieves a lighter piston, and reduces the amount of material potentially contacting the cylinder.

Wiseco Full-Round vs. Strutted
The piston on the right is similar to the full-round on the left, but the piston on the right is made from a strutted forging.

The strutted piston above on the right is sometimes referred to as a slab-sided design, so we asked Fussner for a little insight on the key differences. "The slab-sided forging design is a lighter version. The material on the sides of the forging are moved toward the centerline of the piston, and the skirt panels are directly connected by a fore and aft panel. In addition to the weight savings in the forging, the wrist pin is shorter and lighter. The skirt panels on a slab-sided piston are less likely to deform than the skirt panels on a full round design," he commented.

However, in high power applications, or applications with power adders, that still need a light piston, Wiseco knows some more structural reinforcement is essential. This is where the next version of the strutted design comes in.

Skirt material on the piston is dropped, except for where it’s needed on the thrust axis. With less skirt material, the design of the undercrown needs to be revisited so the piston can maintain shape under stressful conditions. These Wiseco strutted pistons have enhanced pin towers with struts supporting the outer edge of the piston, as well as the piston skirts against the pin towers, and sometimes support between the pin towers.

Wiseco Powersports Piston Wiseco Strutted Piston
Strutted pistons have varying undercrown and skirt design characteristics, but their common trait is piston skirt material only being present on two sides of the piston, instead of all the way around.

"The strutted design is a further design move to create an even lighter piston. A even shorter wrist pin is fitted, and the skirt panel width and length are reduced significantly in this design. These pistons lend themselves to high engine speeds. They're mostly used in modern big bore, short stroke applications, such as motorcycle engines where high engine speeds are common and light weight is important. Most have very short compression heights, which directly reduces the overall piston weight, with many having a very narrow ring belt, with just a top ring and an oil ring," clarifies Fussner.

strutted_design_engineering

This design change allows pistons to shed a lot of weight, and successfully perform under higher engine speed. With less skirt material contacting the cylinder wall, friction is also reduced, creating less drag and contributing to the overall performance. This reduction in material even allowed the piston to get closer to the crankshaft at BDC without interference, which is essential for those running more aggressive bore and stroke combinations.

The video below demonstrates the difference in piston location at BDC with stroker cranks. The strutted forging design allows the piston to safely operate while getting closer to the crank at BDC. Read more on stroker cranks and piston speed here.

Asymmetrical Skirts

An evolution of the strutted design is the asymmetrical skirt design. The characteristics of a piston with asymmetrical skirts are similar to those of standard strutted pistons, but one skirt has substantially less material than the other. Revisiting the major vs. minor thrust load principle we discussed earlier, Wiseco engineers knew the extra material and support is most important on the skirt exposed to the major thrust load. Therefore, the minor thrust side skirt could be trimmed down even further, to promote even less friction and weight, further contributing to performance potential.

Wiseco Standard vs. Racer Elite
The piston on the left has symmetrical skirts, and the right piston is a Racer Elite with asymmetrical skirts. Notice how one skirt has significantly more surface area than the other.

Asymmetrical skirt designs aren’t brand new to the automotive world, but recently, Wiseco introduced this characteristic to powersports with their full-billet, Racer Elite 4-stroke pistons. Get all the details on Racer Elite pistons here.

Wiseco Racer Elite Asymmetrical
This view shows how the minor thrust side skirt (in the foreground) has less material than the major thrust side skirt (in the background).

With more material on side of the piston than the other, the piston is not technically balanced from the true center of the piston. This problem is fixed with an offset pin. Wrist pins on each Wiseco asymmetrical piston are designed to have the correct amount of offset to keep the piston balanced during engine operation.

Wiseco Racer Elite 4-Stroke
The Wiseco Racer Elite uses an offset pin design to keep the piston balanced with asymmetrical skirts.

There are many versions of full-round and strutted piston designs you’ll find looking across multiple applications. Which design is best for your application depends a lot on your particular engine. For Wiseco shelf parts, engineers have already analyzed what works with the applications available in their factory form. For example, strutted designs are the norm across dirt bikes and ATVs, but street bike applications can vary between strutted and full-round.

When it comes to modified engines, both in automotive and powersports markets, all modifcations made must be taken into account when deciding what forging design is best for you. It also comes down to engine builder preference. If you’re not sure what to go with, Wiseco specialists can help decide what design features will be best for your combination.

Wiseco Forged Pistons

Topics: featured, PISTONS 101, Powersports, ENGINE TECH, Tech

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