WRX Aluminium Header Tank Installation – Technical Notes

One of the most overlooked but critical upgrades on the Subaru WRX is replacing the OEM plastic header tank. These tanks are prone to heat-cycling fatigue: the seams weaken, the cap seat distorts, and small cracks form that can lead to coolant loss or, in worst cases, overheating.

Our aluminium replacement is designed as a direct swap, but during installation there are a few details we recommend to ensure a trouble-free result:

Fitment

The tank bolts to the OEM bracket points with no modification, maintaining factory hose routing.

Filler neck accepts the standard Subaru cap (1.1 bar or uprated versions).

Hosework

Bottom hose: We recommend replacing this during installation. It sits low in the system, sees high thermal stress, and is one of the most common failure points on WRX cooling systems. A fresh hose here prevents headaches later.

Bleeding the Cooling System

Refill the system slowly with premixed coolant.

Leave the header cap off during the initial fill.

Turn the heater controls fully to hot to ensure coolant flows through the heater core.

Start the engine and allow it to idle, topping up as the level drops.

As the engine warms, occasional blips of the throttle help purge trapped air.

Switch off and allow the engine to fully cool down.

Once cold, recheck the level, fit the cap, and top up the overflow if necessary.

Outcome

The result is not just cosmetic. A properly installed aluminium header tank increases long-term reliability, stabilises cooling pressure, and gives the cooling system the strength needed for road, fast-road, or track use.