Valve springs for Volkswagen Polo 6N

What the spring actually does

The role of the spring is often summarised as closing the valve. Its true function is more demanding: to maintain constant contact between the tappet and the cam throughout the entire lift, regardless of engine speed. It is this contact that ensures the valve follows the profile defined by the camshaft exactly.

At high revs, the moving parts tend to lose contact with the profile: this isknown as ‘valve lift’. The valve no longer follows the cam; it falls back on its own and bounces off its seat. The consequences range from a loss of power to piston-valve contact. A spring that has lost some of its tension does not necessarily break: it simply lowers the revs at which this phenomenon occurs, and the engine hits a performance ceiling without anyone understanding why.

Original or reinforced

The original spring is designed for the original cam profile and the intended maximum revs. It is the right choice for an engine kept in its original configuration: there is no point in fitting a stiffer spring if nothing else changes.

A reinforced spring is justified as soon as you modify the surrounding components: a camshaft with greater lift, higher operating revs, or heavier valves. It pushes back the point at which the engine starts to struggle, and that is the whole point. There is a real cost involved: higher spring pressure causes the cam and tappet to wear out more quickly, and puts greater strain on the valve guide. Fitted without good reason, reinforced springs shorten the life of the cylinder head assembly without offering any benefit. You should also check compatibility with the valve guides: a reinforced spring often requires guides specifically designed for it.

What to replace with

• The valve guides, which transmit the spring’s force to the valve stem. They become marked by contact with the wedges and must be checked systematically.
• The half-moon keys, those two small conical parts that lock the valve seat onto the valve stem. In practice, they are single-use only: a worn key may fail, and a valve falling into the cylinder will destroy the engine.
• The shims, which are inserted beneath the spring to adjust its seating height, and thus its preload. They are available in various thicknesses and are used to compensate for cylinder head resurfacing.
• Valve stem seals, which it would be foolish not to replace whilst everything is dismantled.

Check before reassembly

Two simple checks distinguish a meticulous reassembly from one carried out by guesswork.

• Free height: measure all the removed springs, side by side. A spring that is noticeably shorter than the others has lost its tension, and the entire set must then be replaced, not just that one.
• Installation height: this is the height of the spring once fitted, with the valve closed. It determines the actual preload and is adjusted using shims. After cylinder head resurfacing or valve seat lapping, the valve rises back into its guide and this height changes: checking it is not just a tuner’s finess, it is what prevents the spring from becoming too slack.

Removing and refitting the valve requires a valve spring compressor, which is a cylinder head tool – not to be confused with a suspension spring compressor. Work on one cylinder at a time, with the piston at top dead centre, to ensure the valve does not fall into the cylinder if a wedge slips out of your hand.