BC3604-1 – Individual Subaru EJ Series Intake Valves (37mm / +1mm) – 1 only valve

For the Subaru builder who measures progress in thousandths of an inch and horsepower in the integrity of every component, the BC3604-1 individual intake valve represents a calculated upgrade. This is not a mass-market part; it is a precision instrument engineered for the specific, demanding environment of a modified Subaru EJ-series engine. When your build plan calls for increased airflow, ported heads, or a higher-lift camshaft profile, standard 36mm intake valves become the bottleneck. The BC3604-1 directly addresses this by providing a high-flow, +1mm oversize valve crafted from premium EV8+ stainless steel. This singular valve is the cornerstone for building asymmetric sets, replacing a damaged unit in a race head, or executing a meticulous +1mm valve job across all cylinders. Every dimension, from stem thickness to margin width, is held to a tolerance that speaks to its performance pedigree, making it an essential component for anyone serious about extracting reliable power from the EJ platform.

Technical Design & Material Science

The BC3604-1 is defined by its application of purpose-driven metallurgy and geometry. It is manufactured from EV8+ stainless steel, a chrome-enriched alloy specifically chosen for its optimal balance of high-temperature tensile strength and corrosion resistance. This material resists the constant pounding and extreme heat cycles of a turbocharged or high-RPM application far better than OEM-style valves, significantly reducing the risk of valve float, tuliping, or stem galling. The +1mm oversize (37mm) head diameter is the primary performance attribute. This increase in valve area directly translates to a greater potential for airflow into the combustion chamber. When paired with a professional multi-angle valve job and matched port work, this valve helps unlock the volumetric efficiency that is critical for making power, particularly in the mid-to-upper RPM range. The stem is precision-machined to the exact OEM specification, ensuring compatibility with standard and aftermarket valve guides and seals without modification.

Precision Engineering for Maximum Flow & Durability

Beyond the raw dimensions, the valve’s profile is optimized for performance. The underhead radius and back-cut angles are designed to promote smooth air entry and reduce turbulence as the charge enters the chamber. The lock groove is machined to exacting standards to ensure positive retention with lash caps or standard retainers. For those pushing boundaries, understanding valve train geometry is critical. A resource like the EngineLabs guide on valvetrain geometry provides essential context for integrating components like this valve into a stable, high-RPM system. The BC3604-1 is built to be the reliable foundation in that equation.

Key Benefits of the BC3604-1 Intake Valve

• Increased Airflow Potential: The 37mm (+1mm) head diameter provides a measurable increase in intake port flow capacity, a prerequisite for higher horsepower builds.
• Premium EV8+ Stainless Steel: Offers superior high-temperature strength and wear resistance compared to stock materials, enhancing valvetrain longevity under stress.
• Precision OEM Fitment: Stem diameter and length match factory specifications, allowing for drop-in installation with appropriate machine work to the seat.
• Individual Valve Solution: Perfect for replacing a single damaged valve, building custom sets, or performing a staggered valve size setup without buying a full, pre-packaged set.
• Optimized Performance Profile: Engineered back-cuts and radii improve flow characteristics, making the most of the increased valve size.
• Foundation for Forced Induction: The enhanced material strength is critical for surviving the increased cylinder pressures and heat of turbocharged or supercharged applications.

Detailed Vehicle Fitment

The BC3604-1 valve is engineered specifically for the Subaru EJ-series engine platform. It is crucial to note that installation requires professional machine work to cut the cylinder head to accept the new, larger 37mm valve seat. This valve is sold individually, allowing for flexible purchasing for repair or custom builds. Primary fitment includes, but is not limited to:

Subaru EJ20: EJ205, EJ207 (STI versions).
Subaru EJ25: EJ255, EJ257 (STI), EJ251, EJ253.
Application Note: This valve is intended for performance use in conjunction with appropriate valve springs, retainers, and machined cylinder heads. Always verify head casting and current valve size before ordering. Compatible with most aftermarket valvetrain components from brands like Brian Crower, GSC, and Supertech.

Technical Specifications

Installation & Final Considerations

The BC3604-1 is a component for a system, not a standalone bolt-on. Its installation is a machine shop procedure, requiring the cylinder head to be disassembled, the old valve seat cut out, and a new, properly-sized seat installed and machined to match the valve’s face angle. This must be followed by a multi-angle valve job to blend the seat into the port for optimal flow. This valve must be paired with upgraded valve springs and retainers capable of handling the slightly increased mass of the larger valve head, especially in high-RPM applications. For the builder seeking a reliable, flow-enhanced foundation for a performance EJ head, the BC3604-1 provides the material integrity and dimensional precision required. It is a testament to the principle that real power gains are built from the bottom up, starting with the fundamental components that control the very breath of the engine.