A valve spring looks like a simple coil. It has to close the valve against the seat every cycle, control the valve train at maximum rpm and do it millions of times without losing tension. When a spring sags or breaks, the valve can float, bounce or drop into the cylinder.

SystemEngine top end
Typical materialsSpring steel, often with a progressive or beehive winding
What to checkFree length, installed height, seat pressure and visible cracks
Technical schematic of Motorcycle valve springs
Fig. 01 — working principleIndoBikes drawing

What it does

The spring closes the valve after the cam lobe passes. It also keeps the follower in contact with the cam, which prevents the valve train from separating at high rpm. Seat pressure is the force holding the valve closed when the valve is on its seat.

Too little seat pressure causes valve float and bounce. Too much increases cam and follower wear and can cause coil bind. The spring must match the cam's lift and the engine's rev range.

How it works

When the cam opens the valve, the spring compresses. When the lobe recedes, the stored energy closes the valve. At high rpm the valve train has inertia; if the spring cannot control it, the valve loses contact with the cam and can land badly on the seat.

Springs are fitted at a measured installed height. Shims under the spring seat change that height and therefore the seat pressure. A spring that is installed too tall is weak; too short can coil-bind before full lift.

Types and variations

  • Single springs. One coil per valve. Common in standard engines.
  • Dual springs. An outer and inner spring. More control and a fail-safe if one breaks.
  • Progressive springs. Variable rate to control both seat and high-lift loads.
  • Beehive springs. A conical shape that reduces moving mass and valve surge.
  • Shimmed springs. Installed height adjusted with shims to set seat pressure.

Wear and failure signs

  • Valve float at high rpm. power falls off abruptly and the engine may misfire.
  • A broken spring makes a metallic rattle and can drop a valve.
  • Sagging springs lose seat pressure, causing poor sealing and hard starting.
  • Worn or cracked retainers and keepers can release the valve.
  • Excessive noise from the valve cover that changes with rpm.
  • A bent valve after a missed shift or over-rev can be a spring control problem.

Choosing a replacement

  • Match the spring to the cam's lift and the engine's maximum rpm. Seat pressure and rate are the key figures.
  • Check installed height and coil-bind clearance. A higher-lift cam may need a different spring or retainer.
  • Replace springs in sets, especially on a high-mileage or tuned engine.
  • Use new retainers and keepers if the old ones show wear or cracks.
  • Do not choose a spring by colour or wire diameter alone. Use the maker's specification.

Fit and compatibility

  • Measure installed height with the valve closed and the retainer fitted. Adjust with shims if specified.
  • Check coil bind at full lift with a feeler gauge. There must be clearance between coils.
  • Check retainer-to-guide clearance at full lift.
  • Use a spring compressor carefully. Bent springs or damaged seals cause leaks.
  • After assembly, turn the engine by hand and confirm the springs do not bind.

Inspection and care

  • Inspect springs whenever the head is off. Look for cracks, rust and uneven coils.
  • Measure free length against the service limit. A spring that is shorter than spec has lost tension.
  • Replace valve stem seals at the same time. Oil leaking past a guide can look like a spring problem.
  • Avoid over-revving a cold engine. Oil flow and clearances are not ready for it.
  • After a cam change, recheck spring pressure and clearance.

Safety and limits

A dropped valve destroys the engine and can lock the rear wheel. A broken spring is a stop-riding fault.

Springs store energy. Use a proper compressor and eye protection.

Do not reuse retainers or keepers that show wear. They are cheap compared with a dropped valve.

Questions

How do I know if my valve springs are worn?

Measure free length and installed height, and check seat pressure with a spring tester. A visual check alone is not enough.

Can I fit a high-lift cam with stock springs?

Usually not. The stock spring may coil-bind or lose control at the higher lift. Check the cam maker's spring recommendation.

Do valve springs need replacing at a fixed interval?

Not usually on a standard road engine. They are replaced when worn, when the head is rebuilt, or when the cam is changed.

What does valve float feel like?

The engine stops pulling cleanly at high rpm, may misfire, and can sound rough. In bad cases a valve can contact the piston.