High Tech Pistons for the Masses – Part 1

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MAHLE Motorsports has a piston lineup they simply call “PowerPak Pistons.” This is an extensive range of extremely high quality “shelf” stock pistons targeted directly at sportsman class racers as well as general high performance enthusiasts. Aside from being high quality and readily available, there are a number of features that set them apart. For example, a number of the piston sets can be specified in either a 2618 aluminum alloy or a 4032 alloy. What’s the difference?

Let’s start at the very beginning: Two very different aluminum forgings arrive at MAHLE’s manufacturing facility. It’s impossible to tell the difference between them visually. One of them is 4032 alloy. The other is 2618 alloy. When it comes to performance, though, the difference becomes evident. 4032 and 2618 both have benefits to the end user, but it is often misunderstood how these alloys can be used correctly. 4032 alloy is crafted with a high silicon content (just over 12%). You can find 4032 in many race and street applications up to the ballpark horsepower of 750. What this means is the aluminum is resistant to expansion. 4032 pistons operate at a tighter piston skirt to cylinder wall clearance and at the same time offer increased stability in the bore. The extra silicon content improves the durability of the piston over the long haul. Essentially, this means an engine with 4032 pistons will maintain ring groove clearance and ring seal for many, many miles.

Pistons manufactured from 4032 alloy can be (and certainly have been) used in performance and race applications with excellent results. But 4032 does have reduced ductility. That means the piston is less resistant to cracking due to extreme impact loads such as detonation or heaven forbid, physical contact with something like a valve.

Because of that, companies like MAHLE Motorsports also offer pistons in another alloy, and that is 2618. The primary difference between a 4032 alloy and a 2618 alloy is the very low silicon content in 2618 (typically under 0.2%). A low silicon content allows the piston be much more malleable. That’s particularly important if the engine encounters detonation. Applications where this can be common include turbocharged engines, supercharged engines and nitrous injected engines.

The added malleability doesn’t come without costs: A 2618 piston has a much greater expansion rate than a 4032 piston (in some cases, as much as 15%). That means you must compensate with larger (cold) piston to wall clearances. As an example, a typical 4032 alloy MAHLE Power Pack piston for a 4.5-inch bore BBC (such as the example shown in the accompanying photos) is installed with a piston to wall figure of between 0.0041-inch to 0.0049-inch. Meanwhile, a similar big bore BBC piston manufactured from 2618 alloy aluminum is installed with a clearance of between 0.0061-inch to 0.0069-inch. By the way, smaller bore small blocks are installed with much tighter piston to wall clearances (many small block 4032 pistons are in the clearance range of 0.0025-0.0033-inch while 2618 pistons for the same engines can be in the clearance range of 0.0046-0.0054-inch). Once warmed to operating temperature, however, both pistons (4032 and 2618) will end up with similar clearances. FYI, you can check MAHLE piston specifications, including piston to wall clearances, online using the following link:

http://www.us.mahle.com/en/motorsports/powerpak-pistons/

Back to alloys for a second: The lower silicon content in a 2618 piston also makes it slightly less wear resistant when compared to a 4032 piston. Obviously, for a racing application, this isn’t much of a consideration because the pistons will be replaced well before they experience significant wear.

MAHLE Motorsports has a solution for piston skirt wear, and that’s a special “Grafal” anti-friction coating on the skirts coupled with hard anodized top ring grooves. The process is essentially a more aggressive means to protect the piston from ring micro welding. The balance of the piston is treated with a phosphate dry-film coating. These are standard features on all MAHLE PowerPak pistons (more below). According to MAHLE, the Grafal skirt coating reduces drag, scuffing, friction and cylinder bore wear and also reduces piston noise. Under very extreme conditions, Grafal can self-sacrifice, which in turn can protect the piston and cylinder wall. Typically, though (under normal circumstances), the coatings are engineered to stay intact.

As mentioned above, MAHLE also incorporates a special phosphate dry film coating on the PowerPak pistons. They use the same coating on specific OEM passenger car pistons right through to heavy-duty diesel engines. That dry-film coating is what gives MAHLE Powerpak pistons their unique grey appearance. When it comes to the phosphate dry-film coating, MAHLE states: “The dry lubricant coating (not to be confused with a thermal film coating) provides a lubricant film in the pin bores and ring grooves until the oiling system of the engine reaches operating pressure; particularly useful during the initial start-up or break-in of engines to protect against galling and micro welding. The piston crown is still machineable.” By the way, machining a phosphate coated piston or scratching the surface while installing pin locks does not have an effect upon piston durability or performance.

Something else you might encounter on certain MAHLE pistons is a special “gold” coating. This is a coating designed for piston use in an aluminum bore environment. Typically, when two similar materials are operated against one another (in this case, an aluminum piston in an aluminum bore), there’s a chance for galling. This is amplified in aluminum silicon bore applications. MAHLE’s Gold Series “Ferroprint” coating is a screen-printed organic resin containing iron and steel particles that is applied to a skirt at a thickness of 12 microns. The coating is resistant to fuel and oil. Under typical conditions the coating doesn’t wear and it’s designed to last the life of the piston.

We’ll end the first part of our MAHLE Motorsports piston series here. Next time around, we’ll dig deep into skirt configurations and we’ll touch on wrist pins. There’s a bunch of info you won’t want to miss. Watch for it - we've got parts two, three and four coming up!

It’s impossible to tell the difference between piston alloys by the naked eye. MAHLE Motorsports offers pistons in both 4032 and 2618 aluminum alloys.

4032 alloy pistons such as this MAHLE Motorsports example can be installed with tighter cold clearances than their 2618 cousins. That’s not the end of the differences, however. The text offers plenty of info on the two alloys and the differences between them.

The grey coloring you see on the MAHLE Motorsports pistons is a dry lubricant phosphate coating. MAHLE uses it because it provides a lubricant film in the pin bores and ring grooves until the oiling system of the engine reaches operating pressure.

Should you need to machine the piston crown on a phosphate coated piston, MAHLE advises it does not present a problem. Ditto if you scratch the coating during pin lock or ring installation. It does not have an effect upon piston durability or performance.

MAHLE PowerPak pistons feature a Grafal skirt coating engineered to reduce drag, scuffing, friction and cylinder bore wear. It also reduces piston noise. Under extreme conditions, Grafal can self-sacrifice, which in turn can protect the piston and cylinder wall.