2618 Pistons: We Dive Into The High-End Alloy With UEM Pistons

In the before times, when you chose a forged piston from the aftermarket, that meant you were probably getting a piston made from 4032 aluminum alloy that had significant strength advantages over a cast hypereutectic piston. However, the past few decades have seen wild advancements in manufacturing and material science that has allowed for more options than ever when it comes to pistons. To shed some light on modern advancement in piston material, we reached out to our friends at UEM Pistons.

One of the biggest advancements over the past 20 years is how common 2618 alloy has become in off-the-shelf applications. While it’s a chicken and the egg situation as to whether 2618 has allowed engines to easily make ridiculous amounts of power, or whether engines making ridiculous amounts of power just needed a stronger piston material than the tried-and-true 4032, the fact is, forged 2618 pistons will handle more cylinder pressure than an equivalent 4032 piston.

2618 is still the king for serious street applications. — Marko Glush, UEM Pistons

Too Good To Be True

So, you might be wondering if 2618 is so awesome, why there would be any debate about using it over 4032. Well, to answer that, first we must look at the two different alloys and identify their differences. Both are made from primarily aluminum, but the differences lie in the other elements present in the alloy. The major difference in the two alloys is the silicon content; 4032 has 12.2-percent silicon and 2618 has only 0.23-percent.

“The reduction in silicon content in the alloy makes 2618 ductile enough to flex without cracking,” says UEM Pistons’ Marko Glush. “Critical areas like the ring lands and skirt struts really benefit from the additional ductility of the material in high cylinder-pressure applications. It’s also better at staying intact with a mechanical collision (like a valve into a piston) should there be a valvetrain failure.”

Piston ring lands can be an area prone to failure with the wrong piston material. With 2618’s reduced silicon content and subsequent increase in ductility, high cylinder pressures are less likely to cause a failure as opposed to 4032.

That difference in silicon also affects some of the “street manners” we’ve come to expect from a forged piston. The lower silicon content drastically affects the overall thermal expansion of the piston, requiring more piston-to-wall clearance than a comparable 4032 piston. On the street, that translates to a much noisier piston until it reaches operating temperature.

That noise can also translate to increased wear on the piston. “That piston ‘slap’ noise — which is due to the increase in piston-to-wall clearance — can result in accelerated wear in the piston skirt. That, in turn, increases the clearances in the bore, and increases the noise and wear,” explains Glush. These facts have led to an overall belief that 2618 pistons aren’t meant for use on the street. Like most things in the world, it’s not that black and white.

This graphic from UEM will help you visualize how the two main forged alloys compare to each other. The major factor in the different alloys is the amount of silicon in the mix. If you pay close attention to the downsides of 2618 on the left, you might notice that modern piston coatings can mitigate most of them.

Taking The Good With The Bad

While there is no getting around the facts of the material science, with today’s modern technology, you can mitigate some of the negatives. “Proper bore finish is mandatory in creating ring rotation for positive ring seal and the ability to hold the correct amount of oil in the crosshatch pattern for skirt lubrication,” says Glush. With today’s advanced honing machines and measuring tools, it’s never been easier to ensure that your bore finish is exactly what you need for a particular application.

Additionally, modern coatings are extremely helpful in reducing wear and extending the lifespan of 2618 pistons. “The recent improvements in coatings to help protect the skirt wear area, such as the Line2Line abradable coating, dramatically increase the service life of the skirts,” Glush explains. “UEM’s latest thermal barrier — UltraThermCM — aids in reflecting the combustion heat off the piston crown. That allows the piston to operate at a cooler temperature, decreasing the amount of material fatigue and maintaining the heat-treat integrity.”

Icon’s crown coating is designed to keep the heat of combustion inside the chamber and out of the piston itself. By keeping the piston cooler, the 2618 aluminum is kept out of a problematic heat range for the material.

While coatings used to exist simply to aid in the break-in process, today’s coatings can actually bridge the gap between the operational envelope between 4032 and 2618 pistons. By enhancing the benefits of a given material, and supplementing its shortcomings, you have piston designs operating and thriving in environments where previously it would have been almost unheard of, primarily making 2618’s use on the street a much more viable option.

Running 2618 On The Street

We’ve said it before, and we’ll continue to say it, it’s never been easier to make crazy power out of an internal combustion engine, with not a lot of money invested. As such, we are seeing power levels on the street that would have only come out of high-end track-only builds only a few years ago. With the increase in performance, comes the use of parts that were never considered “street parts.”

To that end, Glush notes that the market has seen a significant uptick in 2618 use. “We’re seeing it mostly in late-model applications such as GM’s LS and Dodge’s Gen-III Hemi,” says Glush. “As more bolt-on power-adders are available to the home builder, the ability of 2618 to perform well in demanding conditions outweighs the potential shorter service life.”

You can’t tell the difference between 4032 and 2618 just by looking at a piston — especially a fully machined one. The differences in the alloy and their differences in material properties all occur at the molecular level.

If you were to revisit some of the conversations about running 2618 pistons on the street from ten or more years ago, you would get the idea that running such a piston in an engine that saw street duty would lead to catastrophic failure in a very short time span. In reality, the situation was, a majority of the engines on the street didn’t need the benefits 2618 offers high-power engines, and the technology didn’t exist to mitigate the alloy’s drawbacks. And with that, the belief that “2618 doesn’t belong on the street” was born.

As with most things in this hobby, there are many considerations as to what alloy your forged pistons should use, and there are applications where 2618 is simply the wrong choice. “Street-driven, high-mileage engines, that are not inspected on a routine basis aren’t good candidates for a 2618 piston,” explains Glush. “The same goes for marine applications. 4032 alloy is still the correct choice for those. 2618 is still ‘the king’ for serious street applications.”

So, more than a simple “yes” or “no” about being able to run 2618 pistons on the street, like racing, we live in the gray areas. “Not all applications are available in both 4032 or 2618. You may have to choose 2618 because that is all that application is made in,” says Glush. “In a street application with 2618, is the customer OK with the possibility of some start-up piston noise that may not go away when at operation temp? In that case, we’d look at coatings to help reduce wear and possible piston noise on applications with mufflers.”

UEM’s M42 skirt coating is applied at only .0005 (half a thousandth) inch-thick, but can help reduce both noise from piston slap, and skirt wear from cylinder friction. Additionally, there is a specialty coating available from UEM applied by Line2Line coatings, which can help to take up some of the free space in the bore when the engine is cold. Both of those help the forged 2618 piston shown here to be used in an application that might not have typically been recommended for 2618 in times past.

Ultimately, 2618 is best for conditions where incredibly high cylinder pressures will be seen (heavy doses of nitrous, big boost numbers, or incredibly-high compression). The material’s increased ductility over 4032 will help the piston survive in those conditions. As for its use on the street, well, we are seeing more and more engines that fit into the above categories used on the street. So, it only stands to reason that 2618 is becoming more common for “street” engines.

If you need the benefits 2618 offers over 4032, you are likely willing to accept the tradeoffs that come with it. Especially with the modern lineup of coatings that can help mitigate those tradeoffs, using a 2618 piston isn’t as large of a burden as it once was, especially if you can utilize its benefits.

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About the author

Greg Acosta

Greg has spent seventeen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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