WRX Aluminium Header Tank Installation – Technical Notes

WRX Aluminium Header Tank – Installation 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: seams weaken, the cap seat can distort, and small cracks form over time. This can lead to gradual coolant loss or, in worst cases, overheating.

Our aluminium replacement is designed as a direct swap for the factory unit. However, during installation there are a few important details we recommend following to ensure a reliable, trouble-free result.

 

Fitment

 

The tank bolts directly to the OEM mounting points with no modification required

Factory hose routing is retained

The filler neck accepts the standard Subaru radiator cap (1.1 bar or uprated versions)

 

Hosework

 

Draining Coolant – What’s Best?

How the coolant is drained depends on whether you plan to reuse the existing coolant or refill with fresh coolant.

Reusing Existing Coolant

If the coolant is in good condition and will be reused:

Extract coolant using a hand pump through the header tank

Remove only enough coolant to drop the level below the tank outlet

This method keeps the system clean and avoids introducing unnecessary air.

Refilling with Fresh Coolant

If you are replacing the coolant entirely:

It is acceptable to remove the lower radiator hose

Allow the system to drain fully into a suitable container

Refit the hose securely before installing the new header tank

This method drains the system quickly but should only be used when fresh coolant will be added, as it introduces more air into the system.

⚠️ Always ensure the engine is completely cold before draining coolant.

 

Header Tank Bottom Hose (Bottom Outlet)

 

The bottom header tank hose, connected to the lower outlet of the header tank, is a critical part of the cooling system. This hose supplies coolant from the header tank back into the system and operates continuously under temperature and pressure.

On Subaru WRX models, this hose is exposed to:

Repeated heat cycles

Constant pressure variation

Engine vibration

As a result, it is one of the most common failure points in the cooling system. Hoses may soften internally, degrade externally, or fail without obvious visual warning.

We strongly recommend replacing this hose during installation.

With access already available, renewing it now significantly reduces the risk of future coolant loss and cooling system issues.

 

Refilling & Bleeding the Cooling System

(Radiator + Header Tank)

 

Correct filling and bleeding are essential to prevent trapped air pockets, particularly in the cylinder heads and heater circuit.

Initial Fill

Refit all hoses and ensure all clamps are secure

Remove both the radiator cap and header tank cap

Slowly fill the radiator first until completely full

Continue filling via the header tank until the level stabilises

Using a funnel fitted into the header tank filler neck is recommended, as it allows the coolant level to rise within the funnel. This makes it easier to observe cavitation and air bubbles escaping from the system as the engine warms, confirming that trapped air is being purged correctly.

 

Bleeding & Air Removal

 

Set the cabin heater controls to full hot

This opens the heater circuit and allows coolant to circulate through the heater core

Air trapped in the heater circuit is a common cause of unstable coolant temperatures

Start the engine and allow it to idle

Top up the header tank as the level drops

As the engine warms, gentle blips of the throttle can help dislodge remaining air pockets

 

Final Checks

 

Switch off the engine and allow it to cool completely

Once cold:

Recheck and top up the radiator if required

Set the header tank to the correct cold level

Fit both caps

Top up the overflow bottle if necessary

Filling both the radiator and the header tank, combined with running the heater on full hot, ensures trapped air is removed rather than circulating in the system.

 

Outcome

 

The result is not just cosmetic. A properly installed aluminium header tank improves long-term reliability, stabilises cooling system pressure, and provides the durability required for road use, fast-road driving, and track applications.