When it comes to pistons, one of the most common discussions regarding piston material is cast versus forged. Or, debate between the two primary alloys used in forged pistons, 4032 and 2618. However, there is another material debate on the rise lately, and that is billet vs. forged pistons. Over the past half-decade, the popularity of billet pistons has been on the rise, so we reached out to MAHLE Motorsport to find out why that is.

Forging Ahead

First, let’s look at the process of forging a piston. A forged piston starts off as a blank puck of aluminum. That puck is put into a forging press with detailed upper and lower molds. The puck is pressed into these molds under great pressure to form the major details of the piston — the major shapes of the underside of the piston, as well as the skirt and the crown are formed in this pressing.

What the machine spits out is referred to as a forging. A single raw forging can serve as the base for several piston part numbers, but is usually specific to a single piston design. The forging will undergo machining operations to not only define specific features of the piston, but to size it to the exact tolerances required for a high-performance piston.

One of the reasons a forged piston is so strong, is because of how the metal from the raw aluminum slug flows into the mold under all that pressure. The mold is designed to not only create a specific final shape, but to create that specific shape in a specific way. That ensures that the grain structure — the orientation of the crystalline lattice structure at the atomic level of the metal — is arranged in the strongest possible orientation.

Forging is a process where extreme heat and pressure are applied to the base material, forcing it into a mold to take on a very specific shape of the piston being manufactured. This not only minimizes the amount of post-forging machining, but it also creates an improved grain structure within the piston.

Creating Billet Pistons

Billet pistons, on the other hand, are machined entirely from a solid cylindrical slug of extruded aluminum. The extrusion does offer a better grain structure than simply casting a slug of aluminum, but isn’t quite as orderly as a forging. Where a forging only needs machining on certain tolerance-specific parts of the piston (and areas that can be lightened), a billet piston requires 100 percent of the piston to be machined from a solid block of aluminum. This requires significantly more time in CNC machines, making them less cost-effective than a forging-based piston.

However, being entirely machined from a simple cylinder of aluminum offers a billet piston a great advantage over a forged piston: it doesn’t require a forging to be available to make a piston. When supply chain disruptions became the norm instead of the exception, consistent forging availability became an issue. In fact, in our recent Cleveland Vs. Windsor showdown, MAHLE manufactured the Cleveland pistons from billet, simply because the forgings were on extended backorder in the supply chain.

Like a forged piston, billet pistons are available in both 2618 and 4032 alloys, dependant on the application. Once a billet and forged piston are “finished” they both get the same hard-anodized ring lands, MAHLE’s signature gray phosphate piston coating, and Grafal anti-friction skirt coating, making it almost impossible to tell a billet piston from a forged piston when all done, without looking at the underside.

Can you tell which one of these pistons is forged, and which is machined entirely from billet aluminum? The piston on the left is a forged POWERPAK piston for a 347 Windsor, and the piston on the right is for the same engine, but with Cleveland cylinder heads. The only reason for the use of billet in the Cleveland piston was due to a shortage of forgings.

That leaves the question of which piston is stronger. Well, because of the unique grain structure imparted during the forging process, the forged piston does hold an edge in absolute strength. However, for a significant number of applications, the strength difference is negligible.

So really, a billet piston’s advantage comes on the manufacturing side. “We are not ‘locked in’ to a forged structure,” explains MAHLE Motorsport’s Scott Clontz. “We have complete freedom to design to the customers’ needs if we do not have a forging that will meet them.”  Even though significantly more machining time is required, an involved step of the supply chain is eliminated.

That means that for low-volume piston design where a forging doesn’t already exist, it’s more cost-effective overall to just machine the pistons out of a solid billet rather than developing and keeping forgings on hand. Clontz does point out that in a per-unit cost, a billet piston will always be more expensive to manufacture because of the machine time required. However, in cases like ours, where a forging was quite a ways out of stock, but we needed pistons sooner than that, they can be knocked out in a much more timely fashion.

Realistically, choosing billet or forged pistons isn’t something you as a consumer will need to decide, as this is up to the manufacturer. But you can rest assured, whether it’s a billet or a forged piston, you will be getting a killer piston with all the same features, regardless of which manufacturing process creates the piston.

The underside of a forged piston usually requires less machining than a billet piston. However, even forgings can have an almost completely machined underside as well.