Engine Crankshaft

What Is an Engine Crankshaft?

The crankshaft is one of the most fundamental components inside any internal combustion engine. Its primary job is to convert the linear, reciprocating motion of the pistons and connecting rods into the rotational force that ultimately drives the transmission and wheels. Every time a cylinder fires, the force of combustion pushes a piston downward — and it is the crankshaft's offset journals that translate that straight-line push into a continuous spin. Without a properly functioning crankshaft, an engine simply cannot run.

Beyond this basic mechanical role, the crankshaft also plays a direct part in determining an engine's displacement. The stroke — the distance a piston travels from top to bottom dead center — is set by the crankshaft's design. Pairing a longer-stroke crank with a given bore size is a classic way to increase cubic-inch displacement and, in turn, torque output. This is why crankshaft selection is so critical in any engine build, from a street driver to a full competition race motor.

Crankshaft Materials: What to Look For

Not all crankshafts are built the same, and material choice is one of the most important factors when selecting a replacement or upgrade.

• Cast Iron / Cast Steel: Found in most stock OEM engines. Suitable for street-driven applications at factory power levels. An economical option for restorations or budget rebuilds.
• Forged 4140 Steel: A step up from cast, offering significantly greater strength and durability. A solid choice for mild performance builds and street/strip applications where cost is a consideration.
• Forged 4340 Steel: The gold standard for serious performance and racing. A nickel-chromium-molybdenum alloy known for exceptional toughness under high-rpm and high-horsepower loads. Typically features additional finishing processes such as heat treating, shot peening, and micro-polishing.
• Billet Steel: Machined from a solid round bar of steel rather than a forging die. Preferred for custom, short-run, or extreme applications where a dedicated forging is not available.

When Should You Upgrade Your Crankshaft?

A factory crankshaft is engineered to handle factory power levels — nothing more. Once an engine build starts adding power through forced induction, aggressive camshaft profiles, or displacement increases, the stock crank can become the weakest link in the rotating assembly. Upgrading to a forged or billet aftermarket crankshaft provides the margin of safety needed to handle elevated cylinder pressures and increased rpm without risking catastrophic failure.

Top Engine Crankshaft Brands

Speedmaster is one of the most comprehensive aftermarket engine component manufacturers in the performance industry. The brand offers crankshafts in cast nodular iron, 5140 forged steel, 4340 forged steel, and billet 4340 steel — covering everything from budget street builds to top-fuel-level applications across Chevy, Ford, Mopar, and Pontiac platforms. Speedmaster crankshafts undergo precision grinding, heat treating, shot peening, and micro-polishing for tight tolerance control.

Eagle Specialty Products has built a long-standing reputation as a leader in aftermarket rotating assembly components. Eagle offers a tiered crankshaft lineup — from cast steel OEM replacements to forged 4340 steel cranks rated for serious competition use — giving engine builders the flexibility to match their budget and power goals without sacrificing quality.