What Metals Are Engine Pistons Made From?by John Cagney Nash
All modern engine pistons are made from an aluminum alloy. The alloy behaves somewhat differently under use according to how the piston is made, so an understanding of the manufacturing process is important. Until the 1970s, the subject of cast versus forged pistons frequently was a debated topic; since then, advances in technology have made the debate all but redundant to the everyday driver.
Piston Material Evolution
In the original internal combustion engines, steel was used to make pistons. Aluminum alloy took over very early on. The earliest aluminum pistons were subject to considerable expansion and contraction because of operational heat, and the design was evolved so that steel rings -- called struts -- were molded into the walls to reduce the problem. This type of piston was common until the 1960s, when the introduction of silicon into the alloy made strutting redundant. Most modern pistons are made with around 25-percent silicone. The early aluminum-silicone alloy was renowned for its brittleness; accidentally dropping one from bench height usually resulted in a crack that was at best expensive and at worst impossible to repair. The addition of nickel to the alloy reduces the brittleness, but increases the weight-to-mass relationship.
Pistons have nine parts and sections. The piston top properly is called the crown; below this are the ring grooves into which are fitted the piston rings. The raised areas between the ring grooves are called the lands. Below the ring assembly is the piston pin hole. The piston pin -- called the "wrist pin" in industry speak -- passes through this hole and passes through the connecting rod. Around the piston pin are pin bosses that support its ends. The bottom of the piston is called the skirt.
The cast piston is molded from a molten aluminum alloy, which is drawn by vacuum into steel dies; only minimal machining is necessary to finish the resulting piston. The process is called “gravity die casting.” The shape and wall thickness is fully controlled, but the process is expensive.
The forged piston is made first by placing an ingot of warmed aluminum alloy into a female mold; after this, a male ram is forced into the mold to stamp the metal into a piston blank. The blank then undergoes many machining operations; a single forging set-up typically produces a blank that can be machined into numerous piston sizes to suit a wide variety of vehicles.
Casting was the original method of piston manufacture; forging came along later as an alternative. The forging process compresses the alloy’s molecules at the crown, making the metal more dense and, therefore, better able to withstand the extremes of temperature. This is a substantial benefit because the crown is exposed to more heat than any other part of the engine except the spark plug.
Cast pistons are made in intricately shaped dies that determine their shapes both inside and out; this allows for a uniform and constant wall thickness that keeps the mass of the piston to a minimum. The process of setting up the dies is costly, so cast pistons typically are only made for multiple applications and to meet massive production requirements. Forged pistons have a comparatively crude internal shape after being stamped, determined only by the ram being driven into the ingot, then being retracted. This normally means considerable turning and hand-finishing is required. Far tighter tolerances are achieved through this method. For these reasons, performance pistons are almost always forged, while OEM-spec pistons are cast.
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