Ignorance can be bliss when it comes to the origin of pistons before they show up in a neatly packed box on the doorstep. Sometimes it’s just easier to imagine they are forged in an oak tree by mechanically inclined elves then rack simple minds with complex thoughts of five-axis CNC mills and solid modeling software.
But, while elves may have a leg up making toys, cookies, and audible breakfast cereals, piston design and manufacture is best left to professionals like the guys at Ross Racing Pistons in El Segundo, Calif. And, after receiving a set of custom, forged Ross slugs for a supercharged small-block build, we decided to further investigate the mystery of piston origins.
While our trip to Ross “might” have disproved the elf theory – some of us still believe – what we saw there was by no means less impressive.
The 25,000 square foot facility houses rows of high-tech, CNC machining equipment and Ross actually machines the majority of their tooling in-house. Within the cavernous facility, pistons in all stages of production; from pallets of blanks to glistening trays of freshly machined, precision-forged goodness, dot the aisles.These forged blanks are how Ross’ forged pistons begin their lives
Most racers ask only two things before purchasing a piston. “What will the compression ratio be?” and “Is it forged?” What we took away from our visit to Ross is that there is a hell of a lot more to piston development and selection than that.
Forging is a manufacturing process in which a compressive force – a giant hammer of sorts – is used to compact material into a denser, stronger form. So, saying a piston is forged refers to the process by which it was created and not its metallurgical composition. Pistons, save for a few, rare exceptions, are forged from two different aluminum alloys: 4032 and 2618One of the first machining operations a piston undergoes is having this locating groove cut on the lathe. This allows the piston to be precisely centered during the following steps. Next the piston will have it’s wrist-pin bore drilled.
Aluminum of the 4032 variety is a high-silicon, low-expansion alloy. Pistons made from this material can be installed with tighter piston to cylinder-wall clearance (.002”-.003”) due to a lower coefficient of thermal expansion. Tighter piston clearances reduce cylinder wall and ring wear and allow for quieter cold operation but the tradeoff is that 4032 pistons are slightly brittle and tend to crack under severe detonation. This makes 4032-forged pistons better suited for a street/strip car that will see only moderate levels of racing abuse.
For all-out racing applications Ian Akiyama of Ross Racing Pistons recommends 2618 alloy pistons.
“All of our pistons are 2618,” says Akiyama. “2618 is softer and more malleable than 4032 but has a higher tensile strength. This lets the pistons take more abuse and conform to the cylinder wall better under extreme load.”
2618 is a low-silicon, alloy that is used for extreme-duty applications. This stuff expands a lot when it tangles with heat so it’s not the greatest choice for a street engine. Piston to wall clearances need to be in the range of .0035” to .005” and on some occasions even larger. Pistons this loose in the hole are loud and “slap,” especially when cold, and contribute to added wear and oil consumption. This type of piston also has a shorter fatigue life and necessitates replacement more frequently than a 4032 slug. However, this is not a major concern on a race car.
Power adders are another area where different elements of piston design really come into play. While compression ratio reduction is an obvious difference, there are a few subtle engineering tricks that Ross uses to help pistons survive in some of the most inhospitable of boosted environments.Here are two sets of finished pistons awaiting final inspection on the laser micrometer.
“One of the biggest differences between a forced induction piston and an NA piston is weight,” said Akiyama. “People don’t like to hear that but a forced induction piston needs more material to deal with the power and heat. It’s ‘where you place the material’ that is what is critical for a strong piston.”
Extra material isn’t the only barrier to preventing piston meltdown under boost or nitrous.
“We’ll move the ring-land down to keep it away from heat,” said Akiyama. “We’ll also thicken the top ring-land and add an extra .140” of material thickness where the valve pocket gets close to the top ring groove. That’s where they tend to be weakest.”
The Bondo mold on the left was used to construct the forged, dome piston on the right.While that may seem very crude, it actually provides a great representation of the combustion chamber shape for Ross to model.
If there is even the slightest inkling of doubt about what is the right piston for an application, it is always a great idea to consult with a piston manufacturer. And, if it’s determined that no off-the-shelf piston is suitable, Ross can custom engineer one with specific dome profiles, compression heights, valve reliefs and hundreds of other variables. As long as there’s a wrist pin running through the middle, Ross can build it for you, and in a time span of two to three weeks.