A valve is a poppet that seals against a seat in the head. The intake valve opens to let air and fuel in; the exhaust valve opens to let gas out. The exhaust valve runs much hotter and is the one most likely to burn or tulip. Clearance, seat condition and mixture decide how long it lasts.

SystemEngine top end
Typical materialsSteel, stainless or titanium, sometimes sodium-filled
What to checkClearance, seat seal, stem wear and valve face condition
Technical schematic of Motorcycle valves
Fig. 01 — working principleIndoBikes drawing

What it does

The valve opens and closes the gas path into and out of the cylinder. It must seal completely during compression and combustion, then open far enough and long enough to fill or empty the cylinder at speed. The stem guides the valve; the seat seals it; the spring closes it.

The intake valve is cooled by incoming charge. The exhaust valve is cooled mainly through the seat and guide, which is why a tight clearance or a poor seat can burn it. Valve face angle and seat width affect sealing and heat transfer.

How it works

The cam or rocker pushes the valve open. The spring closes it onto the seat. The seat is cut at an angle, usually with a narrower contact band. The valve face mates with that band. A good seal needs concentricity, correct width and a clean surface.

Valve clearance is the small gap in the train that allows for expansion. Too little clearance holds the valve off its seat, so it cannot shed heat and may burn. Too much clearance makes noise, reduces lift and shocks the seat.

Types and variations

  • Intake valves. Usually larger, made of steel or stainless. Cooled by incoming mixture.
  • Exhaust valves. Run hotter. Often use better heat-resistant alloys, sometimes sodium-filled stems.
  • Titanium valves. Light, good for high rpm, but expensive and less tolerant of poor maintenance.
  • Standard and oversize. Oversize valves need seat and head work. They are not a bolt-in upgrade.
  • Valve and seat angles. Different angles and seat widths change flow and sealing. The head shop chooses them to suit the application.

Wear and failure signs

  • Burned exhaust valve. a missing chunk or a black, eroded face. Often caused by tight clearance or lean mixture.
  • Tuliped valve. the head mushrooms from heat and impact. Clearance closes up.
  • Bent valve. timing error, over-rev or piston contact. Compression drops sharply.
  • Receded seat. the valve sinks into the head and clearance closes.
  • Worn stem or guide. oil use, smoke and poor valve control.
  • A ticking noise that changes with temperature can be clearance, a worn rocker or a loose plug.

Choosing a replacement

  • Replace valves in sets when the head is apart, especially exhaust valves on a high-mileage engine.
  • Match the valve material to the seat and the use. A race valve in a road engine is not automatically better.
  • Check stem diameter, length and keeper groove. Small differences matter.
  • If the seats are worn, have them cut and the valves lapped or faced by a head shop.
  • Replace valve seals and check guides at the same time.

Fit and compatibility

  • Set clearance to the manual's figure, cold or hot as specified. Measure with a feeler gauge of the correct range.
  • Check seat concentricity and contact width. A head shop uses a dial gauge and blueing.
  • Check installed spring height and retainer fit if the valves or springs are changed.
  • Turn the engine by hand after timing work to confirm no contact.
  • Do not lap titanium valves with abrasive paste unless the maker allows it.

Inspection and care

  • Check clearance at the recommended interval. A tightening clearance is a warning, not a nuisance.
  • Use the correct fuel octane and mixture. Detonation damages valves and seats.
  • Keep the cooling system and oil clean. Heat is the enemy of the exhaust valve.
  • If compression drops or the engine misfires, test before assuming ignition.
  • After a head rebuild, recheck clearance after the first heat cycle.

Safety and limits

A dropped valve can destroy the piston, head and cylinder. If a valve train noise appears suddenly, stop the engine.

Burned valves can cause a fire risk through leaking combustion gas. Do not ignore a persistent misfire.

Head work needs a proper seat cutter and measurement. Hand lapping alone will not fix a worn seat.

Questions

How can I tell if a valve is burned?

A leak-down test will show leakage into the intake or exhaust. The head has to come off to see the valve. A burned exhaust valve often shows as a steady misfire and low compression on one cylinder.

Do valves need adjustment on a shim-under-bucket engine?

Yes, but adjustment means changing shims, not turning a screw. Measure the clearance, calculate the shim, and fit the correct size.

Can I just lap the valves and reuse them?

Light lapping can restore a seal if the valve and seat are in good condition. Worn seats, burned faces or bent stems need replacement and machining.

Why does valve clearance close up?

Seat recession, valve stretch or tuliping. A closing clearance is a sign of wear and should be checked before it reaches zero.