The cylinder head sits on top of the engine block and forms the combustion chambers for each cylinder in a gasoline engine. The cylinder head holds the intake valves that allow air and fuel to enter the combustion chamber, and the exhaust valves that allow the hot exhaust gases to exit the engine when combustion is complete. The head also holds the spark plugs for each cylinder, and on overhead cam engines one or more camshafts to operate the valves. In pushrod engines, the camshaft is located in the engine block and operates the valves via pushrods and rocker arms.
The key components in an overhead cam cylinder head are:
Intake valves open to allow the air/fuel mixture to flow into the cylinders. In engines with Diriect Injection, only air flows past the intake valves because the fuel is injected directly into the combustion chamber. The exhaust valves open so the pistons can push the burnt exhaust gases out of the cylinders.
Engines may have one intake ane one exhaust valve per cylinder, or they may have two intake valves and one exhaust valve per cylinder (3-valve setup), or they may have to intake and two exhaust vales per cylinder (4-valve setup). More valves increases air flow at higher engine speeds for more power. But the trade-off is less air velocity at low engine speeds which reduces torque at lower engine speeds. Turbocharged engines may have 2-valve, 3-valve or 4-valve heads.
The valves may be operated directly by the overhead cam, or by cam followers that push the valves open when the cam lobe rises. The valves seal compression when closed, so a worn, leaky or burned valve will allow a loss of compression and engine misfire.
As valves accumulate mileage, the machines surface that mates with the valve seat in the cylinder head may become worn, causing a loss of compression. Refacing the valves can restore normal compression. Many valves have a 3-angle finish (30-45-60 degrees) that improves airflow and performance (same for the valve seat in the head).
Valves can be damaged or bent in interference engines if the camshaft timing belt or chain fails and a piston hits a valve. See Interference Engines for more information on this subject. Worn valves can often be resurfaced (along with the valve seats) to restore lost compression, but damaged, bent or burned valves must be replaced.
If you are rebuilding a high mileage engine, inspect the valves for wear and damage and refinish or replace as needed. A valve with a worn or bent stem must be replaced. Most experts recommend replacing exhaust valves in high mileage engines with new ones regardless of their appearance to reduce the risk of valve failure or breakage.
Valve springs can fatigue or weaken with age, causing a loss of compression or high speed misfiring. If a valve spring breaks, the cylinder will lose compression and the valve may hit the piston and be bent or damaged. If the valve drops into the cylinder, it can destroy the piston and cylinder head. Valve springs can be tested with a valve spring tester. Springs that do not meet minimum pressure specifications must be replaced.
See How To Diagnose Weak Valve Springs for more info on this subject.
New valve springs are always recommended if a high mileage engine (over 100,000 miles) is being rebuilt, or if the camshaft is being replaced. Most performance cam kits comes with new stiffer valve springs.
Overhead cam engines have one or two camshafts per cylinder head. If there are two cams, one operates the intake valves and the other operates the exhaust valves. If there is a single overhead cam, the same came operates both the intake and exhaust valves. The cams in many late model engines have Variable Valve Timing which allows valve timing to be advanced or retarded to improve fuel economy, performance and emissions. If the upper valvetrain is starved for proper lubrication, the camshaft lobes or journals may be damaged. If the engine overheats severely, head deflection may bind or even break an overhead camshaft. A slightly worn camshaft may be reground to restore its normal lift, but if the wear is too great the camshaft must be replaced. Installing special aftermarket performance camshafts can improve valve lift and duration to increase horsepower. However, for good drivability and throttle response, the lift and duration of the camshaft must be carefully matched to the application.
Most newer engines have Variable Valve Timing (VVT) to advance or retard cam timing. Retarding the cam typically improves low speed torque and emissions while advancing cam timing improves high speed power and broadens the engine's power band. Some engines only use VVT on the intake cam but most use VVT for both the intake and exhaust cams. VVT systems use a cam phaser mounted on the drive belt or chain end of the cam. The VVT changes cam timing when oil pressure is applied to the phaser. The VVT system is controlled by the engine computer.
VVT phasers can be troublesome if they become clogged with oil varnish deposits or dirt (from not changing the oil often enough), or if they fail to receive adequate oil pressure when commanded by the engine computer. This will usually set a fault code and turn on the Check Engine light.
When a cylinder head requires repairs, it must first be removed from the engine. The head can then be taken to a machine shop for the necessary repairs, or it can be replaced with a rebuilt or used cylinder head. If you can afford it, choose a rebuilt head that has new exhaust valves, all new valve springs and new, reconditioned or relined valve guides. Intake valves can be reground and reused but exhaust valves should usually be replaced. Reusing high mileage valve springs is NOT recommended because weak springs may allow the valves to float (not seat) at higher engine speeds, causing misfiring and a loss of power. Weak springs may also break without warning!
If the cylinder head on your engine requires a valve job, here is what it may involve:
1. Clean the cylinder head to remove dirt and grease.
2. Completely disassemble the head so all of the parts can be inspected, reconditioned or replaced as needed.
3. Inspect the cylinder head casting for cracks or other damage. Penetrating dye can find cracks in aluminum heads, and magnetic crack detection equipment is required for inspecting cast iron heads. Heads can also be pressure tested for leaks in a water tank.
4. If a head is cracked, the cracks can sometimes be repaired by installing locking pins (typically done in cast iron heads with small cracks) or by grinding out the cracks and TIG welding the head (aluminum heads). If the damage is too extensive or too expensive to fix, the head will have to be replaced with a new casting or a used head.
5. Recondition the valve guides. On aluminum heads with powder metal iron or bronze guides, worn guides may be driven out and replaced with new ones. On cast iron heads with integral guides, the old guides may be drilled out to accept guide liners or a new guide. Or guides may be reamed out to accept a new valve with an oversized valve stem.
6. Regrind the faces and tips of the valves as needed (or replace any valves that are badly worn, damaged or cracked).
7. Remachine the valve seats. Most valve seats are cut to 45 degrees. If the seats are damaged or too badly worn to be reground, the seat may be removed and replaced with a new one (aluminum heads) or machined out and replaced (cast iron head).
8. Inspect overhead camshaft(s) and the cam bores in the cylinder head for straightness, wear, damage or cracks. OHC heads often warp in the middle causing cam bore misalignment and uneven cam journal wear. If the cam does not turn freely in the head, either the cam is bent or the head is warped. A broken cam would be another indication of a warped head. If the cam bores are not straight (no more than 0.001 inches of misalignment), the bores can be realigned by hydraulically press straightening the head or by line boring or line honing the bores the head. If the head does not use cam journal bearing inserts and the bores are worn, the head can be machined to accept bearing inserts to restore normal cam clearances. If the cam is bent, replace it.
9. Check and replace valve springs as needed. New springs are recommended for high mileage engines.
10. Resurface the cylinder head to restore a flat, smooth finish that meets head gasket sealing requirements.
11. Reassemble the head, using plenty of assembly lube on the cam, valve followers (if used) and valve stem tips.
12. Reinstall cylinder head on engine with new head gasket and cylinder head bolts (do not reuse torque-to-yield head bolts). Refer to vehicle service literature for proper head bolt torque procedure and specifications, and OHC camshaft alignment and timing procedure. New valve cover, intake and exhaust manifold gaskets should also be used, along with a new timing belt or timing chain set.