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 and Exhaust Valves
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 SpringsHigh pressure springs around each valve pull the valve shut after the camshaft has open the valve. The amoun tof spring tension the spring exerts on the valve when it is closed will vary with the application. Higher revving engines require stiffer valve springs, or even double or triple springs.
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.
Major OHC cylinder head components.
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!
OHC Cylinder Head Repairs
If the cylinder head on your engine requires a valve job, here is what it may involve:
More Cylinder Head Repair Articles:Engine Overhaul
OHC Head Repairs
Head Bolts (Torque-to-yield TTY head bolt installation & removal tips)
Cylinder Head Resurfacing
New Sealing & Resurfacing Requirements for Today's Engines
Preventing Repeat Head Gasket Failures
Cast Iron Crack Repairs
Valve Guide Repairs
Valve Seat Repairs
How To Diagnose Weak Valve Springs
Variable Valve Timing (VVT)
Engine Rebuilding Tips
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