Subaru Engine & Cylinder Head Identification Guide
Subaru utilizes a horizontally opposed “Boxer” engine configuration. Unlike standard inline engines, these powerplants utilize two separate cylinder head castings—identified as Left-Hand (LH) and Right-Hand (RH)—rather than a single cylinder head assembly.
1. FB Engine Series (Naturally Aspirated, DOHC, Chain-Driven)
The FB series replaced the previous EJ architecture, introducing a compact Double Overhead Cam (DOHC) design. These heads utilize rocker-arm valvetrains with hydraulic lash adjusters, eliminating the requirement for manual shim adjustments. All variants include the Active Valve Control System (AVCS) for variable cam timing.
- FB20 / FB20B / FB20D (2.0L NA): Engineered with elongated, narrow intake ports to optimize low-end air velocity and torque. The “D” designated variants feature integrated high-pressure direct injection ports within the combustion chamber.
- Mapped Models: XV / Crosstrek, Impreza, G4 (Impreza Sedan), Forester.
- FB25 (2.5L NA): Utilizes larger combustion chambers and increased intake valve diameters to accommodate the higher displacement volume.
- Mapped Models: Forester, Legacy, Outback.
2. FA Engine Series (Turbocharged / Direct Injection, DOHC)
The FA series is designed for forced induction and higher performance specifications. The aluminum heads feature widened intake ports and reinforced combustion walls to sustain higher boost pressures. This series utilizes Dual AVCS, providing variable timing for both intake and exhaust camshafts.
- FA20DIT (2.0L Turbo): Characterized by square-profile intake ports and reinforced casting zones designed to manage the thermal loads associated with twin-scroll turbocharging.
- Mapped Models: WRX, Forester XT, Levorg, Legacy DIT (JDM).
- FA24DIT (2.4L Turbo): The largest displacement DOHC head in the modern lineup, featuring optimized flow paths for current direct injection technology.
- Mapped Models: Modern Outback, Legacy, WRX.
3. EJ Engine Series (Belt-Driven Architecture)
The EJ series is defined by its use of reinforced rubber timing belts rather than chains. These heads are identified by their characteristic oval-shaped intake ports.
- EJ257 (2.5L Turbo “STI”): Features a shim-over-bucket solid lifter valvetrain for sustained high-RPM operation. Technical note: These castings are susceptible to stress fractures near the internal spark plug threads under extreme thermal loads or high-boost modifications.
- Mapped Models: WRX STI.
- EJ204 / EJ253 (Naturally Aspirated SOHC/DOHC): Standard equipment in various models from the mid-2000s through the early 2010s. SOHC variants utilize a single camshaft per head to actuate both intake and exhaust valves via a rocker-arm assembly.
- Mapped Models: Older years of Impreza, Legacy & Exiga.
Technical Warning: Machining and Resurfacing Constraints
Subaru Boxer cylinder heads have limited tolerance for deck milling or resurfacing. Because the timing chain or belt paths span from a centralized crankshaft across both cylinder heads, removing material from the head surface reduces the distance between the cams and the crank. This can result in excessive timing chain guide slack or misalignment of belt timing marks. If resurfacing exceeds 0.005 inches, oversized (extra-thick) cylinder head gaskets must be installed to maintain correct factory geometry.