BC5139 – Mitsubishi 4B11T (Evo X) Crankshaft – 4340 Billet – 98mm Stroke

In the realm of forced-induction performance, the crankshaft is the spinal column of the engine, translating explosive cylinder pressure into relentless rotational force. For the Mitsubishi 4B11T platform, found in the revered Lancer Evolution X, the quest for ultimate strength and precision leads to one component: the Brian Crower BC5139 4340 Billet Crankshaft. This is not a modified factory piece or a forged alternative; this is a purpose-built, CNC-machined billet artifact engineered for environments where factory tolerances are merely a suggestion. Designed with a 98mm stroke, it forms the foundational cornerstone for high-displacement, high-RPM, and high-boost builds aiming for four-digit horsepower figures while maintaining parasitic loss control and impeccable harmonic balance. For the builder who views the engine as a kinetic sculpture of physics and metallurgy, the BC5139 is the definitive rotating assembly upgrade.

Engineering Superiority: The 4340 Billet Advantage

The core of the BC5139’s dominance lies in its material science and manufacturing origin. Unlike traditional forging, which involves shaping a heated steel blank under immense pressure, billet manufacturing begins with a solid bar of premium AISI 4340 chromoly steel. This “billet” is then meticulously carved into its final form via CNC machining. This process allows for grain structure that is uniform and uninterrupted, following the precise contours of the final part. The result is a crankshaft with superior tensile strength, fatigue resistance, and consistency across every unit. The 4340 alloy itself, nickel-chrome-molybdenum steel, is heat-treated to a optimal hardness, providing an unmatched balance of ductility and ultimate strength to withstand the tremendous bending and torsional loads experienced in high-output 4B11T applications. This combination of material and method creates a component that is fundamentally stronger and more reliable than its OEM or even high-quality forged counterparts.

Precision Balancing & Harmonic Dampening

At operational speeds exceeding 8,000 RPM, harmonic vibrations are not just inefficiencies; they are engine killers. The BC5139 is fully counterweighted and precision balanced to Brian Crower’s exacting standards. Each unit undergoes a two-stage balancing process—first as a standalone crankshaft, and then as part of a matched rotating assembly (when purchased as such). This eliminates inherent imbalances that can lead to catastrophic bearing failure, oiling issues, and structural fatigue. The precise machining of counterweights and journals ensures minimal rotational mass while maintaining absolute strength, reducing parasitic inertia and allowing the engine to rev with a ferocious, yet controlled, responsiveness. This level of harmonic control is non-negotiable for sustained track use or drag racing where engines live at the redline.

Performance Specifications & Build Implications

The Brian Crower BC5139 is specified with a 98mm stroke. This strategic dimension is pivotal for builders targeting a significant displacement increase. When paired with an appropriate overbore cylinder block and longer connecting rods, this stroke facilitates an engine displacement of approximately 2.2 to 2.3 liters, a substantial jump from the factory 2.0L. This increased displacement translates directly to greater low-end torque and volumetric efficiency, providing a broader, more usable powerband. The engine will produce more power at every RPM point, reducing turbo lag and improving drivability, all while maintaining the high-RPM potential the 4B11T head is known for. It is the cornerstone for builds aiming for robust, streetable power or all-out competitive racing output.

Key Benefits of the BC5139 Crankshaft

• Unmatched Strength & Durability: The 4340 billet construction provides the highest possible resistance to flex, twist, and fatigue under extreme boost and RPM.
• Foundation for Large Displacement: The 98mm stroke is the critical first step in building a 2.2L+ 4B11T, dramatically increasing torque and area under the curve.
• Precision Engineered for Balance: CNC machining and dedicated balancing result in smoother operation, reduced bearing wear, and increased engine longevity at high RPM.
• Optimized for Performance Oiling: Designed to work with high-volume oil pumps and improved oiling systems, ensuring critical journal surfaces are always protected.
• Direct OEM Fitment: Drops into the 4B11T block with OEM-spec main and rod journals, though machining for perfect clearance is always recommended.
• Race-Proven Reliability: A staple in time-attack, drag racing, and high-horsepower street Evo X builds around the world.

Vehicle Fitment & Installation Requirements

The Brian Crower BC5139 crankshaft is engineered as a direct replacement component for the Mitsubishi 4B11T engine. However, “direct fit” in a performance context always implies professional assembly and clearancing.

Primary Application:

• 2008-2015 Mitsubishi Lancer Evolution X (all trim levels with the 4B11T engine).

Critical Installation Notes:

• Professional Machining & Assembly Required: This is a blueprint-grade component. The engine block must be align-honed, and the crankshaft must be professionally measured and fitted with appropriate main and rod bearings (e.g., ACL Race Series, King Performance). Clearance checks for rods and pistons are mandatory with the increased stroke.
• Compatible Rotating Assemblies: For optimal performance, pair with Brian Crower or other quality H-beam connecting rods and forged pistons designed specifically for a 98mm stroke application. Using factory rods is not advised for performance builds.
• Oiling System Upgrade: A high-performance build demands a corresponding oiling system. Upgrade to a high-volume/pressure oil pump, consider crankscraper/windage tray, and ensure oil galleries are clear.
• Engine Management: The increased displacement and altered engine dynamics will require retuning of the ECU to optimize air/fuel ratios, ignition timing, and boost control.

The Ultimate Foundation for Your 4B11T Build

Selecting the BC5139 billet crankshaft is a declaration of intent. It signifies a build that prioritizes unshakable foundation over compromise. This component transforms the 4B11T from a potent factory powerplant into a fully realized race-engine platform capable of supporting 800+ horsepower with absolute reliability. The benefits of increased displacement, coupled with the flawless harmonics and heroic strength of a true 4340 billet piece, create a rotational assembly that is as efficient as it is powerful. For comprehensive technical reading on the properties of 4340 steel and its applications in motorsport, refer to this authoritative resource on alloy steel 4340 properties and applications. When your build sheet calls for the best, the Brian Crower BC5139 is the only logical answer for the Evo X’s heart—engineering that turns force into victory.