Cylinder Head and Valvetrain — Everything From Valves to Carbon Buildup

Cylinder Head Construction

The cylinder head sits on top of the engine block and forms the top of the combustion chamber. It contains the intake and exhaust ports, the valve seats and guides, the combustion chamber recess or chamber walls, and in OHC designs, the camshaft bearing bores. The spark plug threads into the head at the top of the combustion chamber. On direct-injected engines, the fuel injector also threads into the head, entering the chamber at a carefully calculated angle.

Most modern cylinder heads are cast aluminum. Aluminum allows for more complex port shapes — longer, more curved runners that optimize airflow velocity at specific RPM ranges — and it is lighter than cast iron. The combustion chamber walls, valve seats, and valve guides are the critical wear surfaces. Aluminum is too soft for direct contact with steel valves, so hardened steel or powdered metal valve seats and bronze or powdered metal valve guides are pressed or cast into the aluminum head.

Cast iron heads are still used on some heavy-duty diesel applications and a few gasoline engines where the higher combustion pressures and temperatures would be more challenging for aluminum. The Subaru EJ series engines used aluminum heads on cast iron blocks — a combination that creates a thermal expansion mismatch that contributes to their head gasket reputation, since the head and block expand and contract at different rates with temperature cycles.

Head combustion chamber design significantly affects engine character. A pent-roof combustion chamber with the spark plug centrally located provides good flame propagation and reduced knock tendency. A bathtub chamber is simpler to manufacture. Hemispherical chambers (HEMI engines) allow large-diameter valves but are more complex to manufacture. The shape of the combustion chamber affects squish — the area between the piston crown and the head surface that creates turbulence in the end gases near the cylinder wall — which helps prevent detonation.

Intake and Exhaust Valves

Intake valves control the entry of the air-fuel mixture (or just air on GDI engines) into the cylinder. Exhaust valves control the exit of burned gases. Both open and close with precise timing controlled by the camshaft, and both must seal completely against their valve seats when closed to maintain compression and prevent exhaust leakage.

Intake valves are larger than exhaust valves on the same engine — larger diameter means more flow area, which improves cylinder filling. Intake valves also run cooler because the incoming air-fuel charge cools them on every intake stroke. Exhaust valves run extremely hot because they open directly into the exhaust stream and only get brief valve-seat contact to conduct heat away. Exhaust valve temperatures can exceed 1,400°F on high-output engines. Some high-performance exhaust valves are hollow and sodium-filled — liquid sodium inside the valve conducts heat from the hot valve head to the cooler valve stem, reducing peak temperatures.

Valve material for intake valves on gasoline engines is typically one-piece steel alloy. Exhaust valves on high-temperature applications use two-piece construction — a heat-resistant alloy on the head end welded to a more machinable steel stem — or a single piece of high-nickel content steel. The contact face of the valve — the narrow angled ring that seals against the seat — is precision-ground to a specific angle, typically 45 degrees, matching the seat angle in the head.

Valve recession is the gradual wear of the valve face and seat that causes the valve to sink deeper into the head over time. As the valve recedes, effective valve lash increases (on OHV engines with adjustable lash) or the valvetrain geometry changes (on engines with hydraulic lash adjusters). Severe valve recession reduces compression and eventually causes valve float even at normal RPM. Leaded fuel prevented valve recession by depositing a thin layer of lead on the seat surfaces — the switch to unleaded fuel in the 1970s required hardened valve seats in all new engines to compensate.

Valve Springs and Retainers

Valve springs keep the valves closed when the camshaft lobe is not pushing on them. The spring tension must be high enough to close the valve quickly and keep it seated against combustion pressure, but not so high that it creates excessive friction and wear on the camshaft lobes and followers. Dual springs — one inside the other — are common on performance engines for higher spring rates and redundancy (if one breaks, the other keeps the valve from dropping into the cylinder).

Valve spring retainers and keepers (cotters) secure the spring to the valve stem. The keeper is a two-piece tapered clip that fits into a groove in the valve stem. The retainer presses against the top of the spring and the keeper locks the retainer to the valve stem. If a keeper breaks or comes out of the groove, the valve drops into the cylinder — engine destruction follows immediately. This is why checking valve spring installed height and keeper condition is critical during any head service.

Spring tension weakens with heat cycling over time. On high-mileage engines, it is common to find springs that are 10-20% below their specified tension. This may not cause immediate valve float at normal driving, but it reduces the safety margin at higher RPM and reduces the force with which the valve seals against the seat. When doing a head rebuild on a high-mileage engine, replace all valve springs — they are inexpensive compared to the labor cost of going back in to fix a spring failure.

OHC vs OHV Design

The fundamental difference between OHV (overhead valve) and OHC (overhead cam) is where the camshaft lives and how motion is transferred from the cam to the valves.

In an OHV engine — also called a pushrod engine — the camshaft is mounted in the engine block. Lobes on the camshaft push on lifters (also called tappets), which push up on long steel rods (pushrods), which pivot on rocker arms at the top of the engine, which push down on the valve stems. The GM LS V8, Chrysler HEMI V8, and most American V8 engines of the last 50 years use OHV design. The advantages: the camshaft is in the block where it is easy to lubricate, the valvetrain is compact (low profile engine), and the engine can be made shorter. The disadvantages: pushrods add mass and flex to the valvetrain, which limits maximum RPM compared to OHC designs. The long pushrod path also means more components that must be lashed correctly and more potential failure points.

In an OHC (overhead cam) engine, the camshaft is mounted in or on the cylinder head. There are no pushrods. The cam lobes act directly on rocker arms or cam followers that contact the valve stems. SOHC (single overhead cam) has one camshaft per head that operates both intake and exhaust valves. DOHC (dual overhead cam) has two camshafts per head — one for intake, one for exhaust. DOHC allows independent control of intake and exhaust timing, which is why virtually all modern VVT systems use DOHC.

OHC design advantages: the shorter valvetrain path reduces reciprocating mass, allowing higher RPM capability. Cam-in-head positioning allows more precise valve timing with less deflection in the train. DOHC enables VVT on intake and exhaust independently. Disadvantages: the head is taller and heavier. The timing drive — chain, belt, or gear — must reach from the crankshaft all the way up to camshafts in the head, making the chain or belt longer and more complex on multi-cylinder engines.

Rocker Arms and Followers

Rocker arms and cam followers are the components between the camshaft lobe and the valve stem. On OHV engines, rocker arms pivot on a fulcrum (stud-mounted or shaft-mounted) and transfer the pushrod's upward motion into downward pressure on the valve stem. On OHC engines, the rocker arm or follower is positioned directly between the cam lobe above and the valve stem below — no pushrod.

Hydraulic lash adjusters (HLA) or hydraulic lifters automatically maintain zero valve lash. They use engine oil pressure to expand and fill any gap in the valvetrain. This eliminates the need for periodic valve adjustments but requires adequate oil pressure and clean oil to function correctly. A sticky or collapsed hydraulic lash adjuster causes a ticking or tapping noise — typically loudest on cold startup and sometimes clearing as oil warms and flows more freely. Persistent lifter tick after warm-up usually means the lifter is not holding pressure due to wear, varnish buildup, or low oil pressure.

Roller followers — followers with a small roller bearing that contacts the cam lobe instead of a sliding contact — dramatically reduce friction compared to flat-faced followers. Most modern engines use roller followers throughout. The roller eliminates the hydrodynamic film requirement of flat followers, which is why modern engines can use lower-viscosity oils (0W-20) without the cam lobe wear that would have occurred with such thin oil on older flat-follower designs.

Valve Stem Seals

Valve stem seals control oil consumption through the valve guides. The valve stems must be lubricated where they slide through the valve guides — but you do not want excess oil in the combustion chamber. Valve stem seals — small rubber or PTFE seals that fit over the valve stems at the top of the guides — restrict oil flow to a controlled amount.

Intake valve stem seals are more prone to oil consumption than exhaust. On the intake stroke, the manifold vacuum above the intake valve creates a partial vacuum that pulls oil down past the valve stem seal into the combustion chamber. Exhaust valve stem seals also allow some oil through, and the exhaust manifold heat accelerates seal degradation over time.

The symptom of worn valve stem seals — blue smoke on startup and on overrun — is distinct from worn piston ring symptoms. At startup, oil that has pooled on the valve stems overnight gets pulled into the combustion chamber on the first few intake strokes, causing a puff of blue smoke that clears quickly. On overrun (closed throttle, engine braking at higher RPM), the high manifold vacuum pulls oil aggressively through the worn seals.

Valve stem seal replacement can be done without removing the cylinder head on many engines using a specialized tool that keeps the valves held closed with compressed air while the keeper and retainer are removed. This is a significant labor savings compared to a full head remove and disassemble. The air tool threads into the spark plug hole and pressurizes the cylinder to hold the valve against its seat while the spring is compressed and the keepers removed.

Carbon Buildup on GDI Engines

This deserves its own article — and gets one (see the GDI carbon buildup article) — but the fundamentals belong here. In a port-injected engine, the fuel injector sprays gasoline into the intake port, coating the back of the intake valve with every injection event. That fuel spray washes away any oil vapor deposits before they can bake onto the valve surface. Intake valves on port-injected engines stay relatively clean throughout their service life.

GDI (gasoline direct injection) engines inject fuel directly into the combustion chamber, downstream of the intake valves. The intake valves never see fuel spray. PCV system gases — oil vapor from crankcase ventilation — continuously coat the back of the intake valves. Combustion heat on every power stroke partially bakes that oil film. Over 50,000 to 100,000 miles, the deposits accumulate into hard, irregular carbon nodules that can restrict airflow into the cylinder by 30-50% in severe cases.

The symptoms are rough idle, misfires (typically P030X codes), reduced power under load, and poor fuel economy. The carbon deposits disrupt airflow into the cylinder, reduce effective compression, and can cause hot spots that contribute to detonation. Some engines — particularly the Ford EcoBoost 2.0 and 2.3L, the VW/Audi 2.0 TSI, and early Hyundai 2.4 GDI engines — are known for accumulating significant carbon deposits by 60,000-80,000 miles.

Treatment is walnut shell blasting — a process where walnut shell media is blasted through the intake ports with the valves closed, abrading the carbon deposits off the valve faces and port walls without damaging the aluminum ports or the valve surfaces. The procedure requires removal of the intake manifold for access to the ports. Some shops use chemical treatments sprayed into the intake, but these are generally less effective on heavy deposits. The only permanent prevention for dedicated GDI engines is port injection added to the GDI system — a dual-injection strategy used by BMW and others that restores the fuel washing of the intake valves.

Frequently Asked Questions