Cylinder Heads and Related Parts

The primary function of the cylinder head is to:

Support the crankshaft Close the top of the cylinders, form part of the combustion chamber and support valves and related components Store engine oil only Hold the engine mounts

Most modern cylinder heads are manufactured from:

Plastic Cast iron or aluminum alloys depending on application Glass Wood composites

Combustion chamber shape in the head is important because it:

Determines tire size Affects compression ratio, mixture motion, combustion speed and emissions Has no effect on engine operation Only affects paint colour

The ILM explains that valve seats provide:

Decoration around the valves A hardened sealing surface where valves contact the head A location for spark plug threads Oil storage for the valve train

Valve guides are used to:

Control coolant flow Guide and support valve stems as they move up and down Drive the camshaft Support the crankshaft mains

Valve stem seals are installed to:

Increase oil flow into the combustion chamber Control the amount of oil that passes down the valve stem into the guide Seal combustion gases at the valve face Replace head gaskets

According to the ILM, intake valves are often larger in diameter than exhaust valves because:

Exhaust gas is lighter than intake charge A larger opening helps fill the cylinder with fresh mixture or air They operate at higher temperature They require less flow

Exhaust valves typically run hotter than intake valves because they:

Are cooled directly by incoming air–fuel mixture Are exposed to hot exhaust gases leaving the combustion chamber Are made of cooler materials Operate only at idle

Multi-valve heads (such as four valves per cylinder) are used because they:

Reduce airflow Improve breathing, efficiency and power by providing more total valve area Eliminate the need for camshafts Reduce manufacturing cost only

The head gasket is designed to:

Lubricate the valves Seal combustion pressure, coolant and oil passages between block and head Control valve timing Store fuel in the head

Torque-to-yield (TTY) head bolts are designed to:

Be reused indefinitely Stretch into a controlled elastic range when torqued to specification and are often replaced, not reused Require no torque specification Be stronger when over-tightened repeatedly

Cylinder head warpage can result from:

Low engine temperature Overheating and uneven thermal expansion Clean coolant Correct torque procedures

Signs of a leaking head gasket described in the ILM include:

Higher fuel economy only Coolant in the oil, white exhaust smoke, compression loss or combustion gases in coolant Dry spark plugs Smooth idle with no coolant loss

Valve spring pressure is important because it:

Controls coolant flow Keeps valves seated and follows cam motion without float at operating speeds Prevents oil from reaching the valve stem Determines compression ratio

Valve keepers (collets) and retainers are used together to:

Hold the valve head to the stem Secure the valve spring to the valve stem groove Adjust valve lash Seal oil drain-back holes

When reconditioning cylinder heads, the ILM notes that machinists may:

Replace tires and wheels Resurface head faces, replace or recondition valves and seats, and replace guides as needed Adjust wheel alignment Service transmissions

Aluminum cylinder heads often use inserted valve seats because:

Aluminum provides sufficient hardness on its own Hardened seat inserts handle the high temperature and wear better than the aluminum casting They are easier to paint They reduce combustion temperature

A 'cracked head' may cause:

Improved emissions Coolant loss, oil contamination and compression leakage Higher compression ratio with no side effects Better starting

From a parts technician’s standpoint, common cylinder head-related parts include:

Only spark plugs Complete heads, valve train components, head gaskets, head bolt sets, intake and exhaust gaskets Only fuel tanks Only radiators

The ILM explains that different head gasket designs (multi-layer steel vs composite) are chosen based on:

Vehicle colour Engine design, surface finish requirements, clamping loads and durability needs Radio options Tire size

Overhead cam cylinder heads often incorporate:

Camshaft(s), cam bearings, followers and sometimes variable valve timing hardware directly in the head casting No moving parts The crankshaft only Only exhaust manifolds

Valve recession into the seat can lead to:

Increased valve lash and higher compression Loss of valve clearance, poor sealing and burnt valves Improved fuel economy Reduced exhaust temperature

Coolant passages in the cylinder head are important to:

Provide mounting for accessories only Carry heat away from combustion areas and valve bridges Store extra oil Control ignition timing

In diesel engines, some cylinder heads incorporate pre-combustion chambers or swirl chambers to:

Store coolant Improve mixture formation and combustion at lower injection pressures Hold spark plugs Support exhaust manifolds

Head bolt tightening patterns are specified so that:

Warping of the head and uneven gasket loading is minimized The paint cures evenly Coolant flows more quickly Valve timing is automatically set

The ILM notes that resurfacing a cylinder head affects:

Only the appearance Combustion chamber volume, compression ratio and cam timing (on some OHC engines) Only exhaust noise Only fuel tank capacity

Intake and exhaust manifolds attach to the head to:

Support the crankshaft Distribute intake charge to cylinders and carry exhaust gases away Pump coolant Control oil level

When ordering a replacement cylinder head, parts technicians must confirm:

Only the colour of the valve cover Engine family/code, casting number, number of valves, port layout and emissions equipment provisions Only tire size Only transmission type

The ILM explains that some engines use separate cam carrier assemblies which:

Hold crankshaft bearings Bolt to the head and contain the camshaft journals and caps Replace pistons Support the flywheel

Valve seat angle and width are important because they:

Determine coolant flow only Affect sealing, heat transfer and valve life Determine oil viscosity Set spark plug gap

Excessive oil consumption that is not from rings may be caused by:

Dry air filters Worn valve guides or defective stem seals Correct valve lash High-quality fuel

Pressure testing a cylinder head is used to:

Measure compression ratio Check for internal cracks or porosity that allow coolant leakage Adjust valve timing Determine oil capacity

Double overhead cam multi-valve heads may require parts technicians to supply:

Only one rocker arm Multiple camshafts, unique followers, different intake/exhaust valves and complex gasket sets Only spark plug wires Only crankshafts

The ILM stresses that mixing head bolts of different lengths or types:

Is acceptable if torque is correct Can lead to uneven clamping and gasket failure Improves gasket sealing Has no effect on service life