BC6618 – Subaru BRZ, Scion FR-S, Toyota GT86 – ProH625+ w/ARP Custom Age 625+ Fasteners

Engineering Uncompromised Reliability: The BC6618 ProH625+ Connecting Rods

For the dedicated builder of the Subaru BRZ, Scion FR-S, or Toyota GT86 platform, the quest for power is intrinsically linked to the pursuit of foundational strength. The factory FA20/4U-GSE engine’s connecting rods represent a known threshold, a bottleneck that must be addressed to safely harness the potential of forced induction, aggressive nitrous, or high-RPM naturally aspirated builds. The BC6618 ProH625+ Connecting Rod Kit is not merely an upgrade; it is a fundamental re-engineering of this critical link between piston and crankshaft. Manufactured by Brian Crower with a singular focus on durability and precision, this kit incorporates ARP Custom Age 625+® fasteners as standard, establishing a new benchmark for streetable performance and extreme-duty resilience in a direct-fit package. This component is engineered for those who measure reliability in horsepower increments and define safety margins by the quality of their rotating assembly.

Material Science and Precision Manufacturing

ProH625+ Alloy: The Core of Endurance

The defining characteristic of the BC6618 rod set is its construction from Brian Crower’s proprietary ProH625+ alloy. This is a vacuum-melted, high-carbon steel forging that undergoes a specific thermal cycling process to achieve an optimal balance of tensile strength, fatigue resistance, and impact toughness. The “625” denotes a nominal tensile strength of 625 ksi (approximately 4300 MPa), a figure that significantly surpasses that of 4340 chromoly and even many “2000 series” alloys. The result is a rod capable of withstanding the immense compressive and tensile loads generated in high-output FA20 applications—such as those from turbocharging, supercharging, or sustained high-RPM operation—without the potential for brittle fracture associated with some ultra-high-strength materials. This makes the ProH625+ an ideal choice for builds targeting 500+ wheel horsepower while maintaining daily-driver characteristics.

Machining Integrity and ARP Fastener Synergy

Strength is irrelevant without precision. Each BC6618 rod is CNC-machined to exacting tolerances for critical dimensions: center-to-center length, big end bore roundness, and small end pin fit. This ensures perfect piston alignment and oil clearance consistency across all four cylinders, which is paramount for balanced cylinder pressures and long-term bearing life. The kit’s inclusion of ARP Custom Age 625+ rod bolts is not an afterthought but a core part of its engineering. These fasteners are manufactured from a nickel-chromium-moly alloy and age-hardened to provide exceptional clamp load retention and resistance to fatigue under dynamic loads. The use of these industry-leading bolts guarantees the big end of the rod remains perfectly circular under extreme combustion pressures, eliminating a primary cause of spun bearings.

Performance Advantages and Build Benefits

Integrating the BC6618 ProH625+ rods transforms the FA20’s bottom-end from a point of concern to a pillar of confidence. The benefits are systemic, affecting every aspect of engine performance and longevity.

• Unmatched Tensile and Fatigue Strength: The ProH625+ alloy provides a safety margin far beyond OEM and standard aftermarket rods, directly enabling higher boost levels, nitrous shots, and elevated redlines.
• Perfect OEM Dimensional Fitment: Designed as a true drop-in component for the FA20, these rods maintain stock bearing sizes, rod length, and pin diameter, simplifying the build process.
• Weight-Matched Set: Each set is weight-matched to within ±1 gram, drastically reducing rotating mass imbalance. This minimizes parasitic vibrations, reduces stress on the crankshaft, and contributes to smoother engine operation.
• Proven ARP Fastener Technology: The included ARP 625+ bolts offer superior resistance to stretch and fatigue compared to standard 8740 or even 2000-series bolts, ensuring consistent bearing crush and big-end integrity.
• Enhanced Oil Flow & Cooling: The beam profile is designed to promote optimal oil flow to the piston pin and underside of the piston, aiding in critical heat dissipation in high-stress environments.
• Foundation for Future Power: Installing these rods during any engine rebuild or build creates a platform capable of supporting future power adders without the need for another teardown.

Vehicle Fitment and Application Notes

Direct Application & Critical Compatibility

The BC6618 kit is engineered for a direct, precision fit in the following applications. It is imperative that during installation, all clearances—especially connecting rod side clearance and bearing oil clearance—are measured and verified by a qualified machinist or engine builder using precision tools.

Primary Fitment:

• 2013-2020 Subaru BRZ (FA20 D-4S Engine)
• 2013-2016 Scion FR-S (FA20 Engine)
• 2013-2020 Toyota GT86 (4U-GSE/FA20 Engine)
• 2017+ Toyota 86 (FA20 Engine)

Installation Consideration: While these rods are a direct replacement, their installation is part of a full engine build or rebuild process. This includes, but is not limited to, crankshaft inspection/machining, bearing selection, piston pin fitting, and complete balancing of the rotating assembly. For an in-depth technical analysis of the FA20’s strengths and limitations that underscores the importance of a robust rotating assembly, refer to this authoritative resource on Subaru FA series engine engineering.

Technical Specifications

Final Mechanic’s Note: The BC6618 with ARP 625+ fasteners represents the definitive solution for the performance-oriented FA20 builder. It eliminates the OEM rod as the weakest link in the powertrain, allowing you to focus on tuning, airflow, and fuel delivery with absolute confidence in the integrity of your rotating assembly. This is not a part you compromise on; it is the foundation upon which reliable, extreme performance is built.