Water pipe for Audi TT (8N)

What is a water pipe?

In a cooling system, not everything is flexible. Between the cylinder head, the water pump, the thermostat housing and the radiator, the coolant flows through rigid fittings which define the system’s geometry and to which the hoses are connected. These are known as water pipes, or cylinder head hose connections.

They are found in three main locations: on the side or rear of the cylinder head, on the water pump housing, and at the distribution point – in the form of an H- or T-shaped pipe – where several circuits converge, including the cabin heating system.

Why they eventually crack

Most of these fittings are made of glass-fibre-reinforced polyamide, chosen for its lightness and stability. Over the years, repeated heating cycles and constant contact with the coolant cause the material to become brittle. The crack usually starts at the hose connection or at a rounded edge, where the material is under the greatest stress.

The symptom is typical: a leak when the engine is cold, upon starting, which stops once the engine has warmed up because thermal expansion closes the crack. Many owners spend a long time searching for a leak that they never see when the engine is hot. A second clue: a hose that snaps clean through the first time it is removed, even though it appeared sound, a sign that the material has already become brittle throughout the entire circuit.

Original plastic or aluminium fitting

Replacing it with an identical polyamide part restores the original component and its exact fit. This is the choice that ensures compliance and causes the fewest issues during reassembly.

The aluminium fitting, on the other hand, solves the problem once and for all: it does not become brittle. It does, however, require a little care, as it does not tolerate excessive tightening of a clamp and conducts heat, which can cause the hose to stiffen where it comes into contact with it. On a vehicle that is in regular use, this is generally the best compromise. For a car being prepared for a trade show, the original plastic hose remains the best option.

What else needs replacing at the same time

• The O-rings on the pipe, without fail. An O-ring that has been compressed for years will not return to its original shape, and will leak on a new part.
• The clamps, especially worn spring clamps or screw clamps that have already been reused, which leave marks on the hose.
• The hoses that connect to the pipe, if they feel hard to the touch. They have been through the same heating cycles.

When reassembling, bleed the system thoroughly: an air pocket at the highest point causes localised overheating and puts strain on neighbouring fittings, which are often the next to fail.

Water pipe and intake pipe: two different components

The term ‘pipe’ refers to two unrelated components. The water pipe, the subject of this page, forms part of the cooling system and carries liquid. The intake pipe, or intake manifold, channels air to the cylinders and is part of the carburettor or fuel injection system.

If you’re looking for a fitting that’s leaking fluid, you’ve come to the right place. If you’re looking for the part between the carburettor or throttle body and the cylinder head, you’ll need to check the intake pipe range.