Coolant Tanks

Coolant tanks hold the extra coolant that a cooling system needs as it heats and cools, and give trapped air a place to separate out. You will find aluminium expansion tanks for specific cars, universal reservoirs and overflow bottles, filler necks, and ice tanks for water-to-air intercooler systems.

There are two kinds, and they are not interchangeable. An overflow, or recovery, bottle is unpressurised: coolant pushed past the radiator cap as it expands collects there and is drawn back as the engine cools. An expansion tank, also called a header or surge tank, is part of the pressurised circuit and usually carries the pressure cap itself.

The tank’s air space absorbs the coolant’s expansion, and because coolant moves slowly through it, entrained air separates and stays at the top. That matters, since bubbles insulate hot spots in the head, encourage pump cavitation and make temperature readings jump.

Identify the system type first, then the tank.

• Vehicle-specific tanks: aluminium expansion and overflow tanks that use the factory mounting points and hose connections, in silver, black, polished or red.
• Universal bottles: aluminium overflow bottles of 1 litre in red, black or blue, a polished 1-quart version and a 0.9-litre tank, plus universal aluminium reservoirs.
• Filler necks: machined aluminium necks in 32 and 42 mm add a filling point and cap seat to a custom tank or radiator. Match the cap to the neck size.
• Ice tanks: 2.5- and 5.0-gallon tanks for water-to-air intercoolers hold iced water as a heat sink for short, full-load runs, where tank capacity sets how long the cooling lasts.
• Power steering: an aluminium power steering reservoir is listed here as well.

Mount an expansion tank above the highest coolant passage in the engine, so air collects in the tank rather than in the head. On a pressurised tank, the small vent lines from the radiator top and the engine’s high points enter at the top, and the larger outlet feeds the pump inlet. Fill only to the cold mark, because the air space is part of how the tank works.

Never open a pressurised tank while it is hot. Check welds, brackets and hose spigots for cracks at each service, as vibration finds any weak point in a rigid aluminium tank.