What Does a Header Tank Actually Do?
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Keeping your cooling system reliable isn’t just about radiators and hoses — the header tank plays a key role. By managing air, pressure, and coolant supply, it makes sure the system runs smoothly whether you’re daily driving or pushing hard on track.
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🌡 Pressure Control
As the engine heats up, coolant expands. A header tank gives that expansion somewhere to go, and the pressure cap makes sure the system runs at the correct pressure.
Too little pressure → coolant boils early.
Too much pressure → hoses or seals can fail.
The header tank and cap balance this perfectly, so the system stays safe and efficient.
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🌬 Passive Air Collection
Air in the cooling system causes hot spots, cavitation, and unstable temperature readings. A header tank is mounted at the highest point so that trapped air can migrate upward and collect there.
This is a passive process — a header tank does not actively separate air from coolant while the engine is running.
A header tank relies on gravity to collect air, while a swirl pot uses controlled coolant flow and centrifugal forces to actively separate and remove air.
Breather / Overflow Outlet (at the filler neck):
The small pipe lets excess air and coolant escape safely, usually into an expansion bottle.
Return / Bleed Outlet (just below the neck):
Typically 8–10 mm, this outlet allows a constant trickle of coolant — and any small air bubbles from the engine — to flow back into the tank while running.
Together, these outlets keep the system self-bleeding and stable under pressure.
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🔧 Coolant Supply
Most header tanks also feature a larger outlet at the base. This feeds coolant directly back into the system, keeping the circuit topped up even under heavy load when the pump is working hard. It makes the header tank an active part of the pressurised cooling circuit, not just a reservoir.
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🔨 Built to Last
OEM plastic tanks are fine when new, but they become brittle over time and can split without warning. An aluminium header tank, TIG-welded and pressure-tested, is built to handle years of heat cycles, track abuse, and daily driving without letting you down.
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✅ With pressure control, continuous air bleeding, and reliable coolant supply, a properly designed header tank is a small upgrade that makes a big difference.