Mazda Training manual - part 240

 
4 – VALVE TRAIN 

 

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Piston Engine Fundamentals

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The cylinder head includes spark plug mounting holes, valve ports and 
seats, and coolant and oil passages. 

 
 
 

Sealing

 

 

The cylinder head gasket, shown in Figure 30, seals the combustion 
chambers. The gasket is usually made of steel coated with a softer sealing 
material. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Because this seal is so important to engine operation, the head gasket 
must be replaced whenever the cylinder head is removed and replaced. In 
addition, the sequence and procedure for tightening the cylinder 
head bolts is critical to good sealing.

 

 
 
 
 
 
 
 
 
 
 
 

FIGURE 30. The 
cylinder head 
gasket seals the 
combustion 
chambers and 
coolant 
passages. 
 

Left cylinder 
Head gasket 
for V-6 
engine 

Cylinder 
opening 

Identifying 
mark for left 
cylinder head

 

 

 
4 – VALVE TRAIN 

 

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Piston Engine Fundamentals

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FIGURE 31. 
Valves seal 
against a seat in 
the cylinder 
head. 
 

Keeper 
groove

 

Stem

 

Face

 

Margin

 

Head

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

VALVES

 

 
As shown in Figure 31, a valve has a round head with a tapered face that 
seals against a seat in the cylinder head. Because of its stem-and-head 
construction, the valve is sometimes called a mushroom valve. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Components

 

 

The head of the valve is the larger end that seals the valve port. The 
surface of the head that seals the port is called the face, and it is tapered 
at an angle. The valve seat in the cylinder head is ground to match the 
angle of the valve face to provide a tight seal. 

 

The valve margin is the thickness of the valve head. On some valves, the 
margin can be ground to form a new face finish so the valve can be 
reused after wearing. The margin also lets the valve transfer some of the 
heat created in the combustion chamber. 
 

 
4 – VALVE TRAIN 

 

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Piston Engine Fundamentals

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The valve stem is the long, narrow part above the head. The stem has a groove at the end 
for keepers. The valve spring pulls the head of the valve up to close the port. The spring is 
held in place by an upper spring seat and the keepers. The stem fits inside the valve guide, 
which holds the valve in the cylinder head. 

 
 
 

Construction

 

 

Valves are made of special high-strength steel that resists heat. Heat resistance is not quite 
so important to intake valves because they get a cool “bath” every time a fresh air-fuel 
mixture enters the cylinder. However, exhaust valves receive heat blasts of up to 4000 
degrees Fahrenheit every time they open. 

 

As protection from these extreme temperatures, exhaust valves have two ways of 
transferring the heat from the exhaust gases. First, when the valve is seated, some of the 
heat is transferred from the valve head to the seat. The seat transfers the heat to the 
cylinder head, which has coolant passages to control temperature. Second, the valve stem 
transfers heat to the valve guide, which is located in the cylinder head. 

 

The width of the valve face and margin are critical. The face must be perfectly uniform and 
wide enough to seal tightly and transfer heat. The margin must be large enough for the 
valve head to absorb and hold the heat until it can be transferred. 

 
 
 

Multi-Valve Design

 

 

Mazda engines use three or four valves per cylinder because multiple valves are more 
precise and efficient. A three-valve design typically uses two valves for intake and one 
valve for exhaust. A four-valve design uses two valves each for intake and exhaust. 

 

 
4 – VALVE TRAIN 

 

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Piston Engine Fundamentals

TC010-05-01S

 

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

FIGURE 32. Each 
valve seat has 
several 
machined 
surfaces to make 
a tight seal. 

 

Upper 
clearance

 

Valve seat

 

Lower 
clearance

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

VALVE SEATS

 

 
The valve seat is made of very hard metal so it can stand up to constant 
hammering as the valve opens and closes. When the engine is running at 
high speed, the valves may open and close up to 25 times per second. 
 
In addition, the intense heat in the combustion chamber and the 
chemical properties of the exhaust gases can cause corrosion. So the 
valve seat must also be resistant to heat and corrosion. 
 
If the cylinder head metal meets these requirements, the valve seat may 
be machined directly into the head. If the cylinder head metal is not 
strong enough to withstand the hammering, corrosion, and heat, the 
valve seat may be a hardened steel insert that is pressed into the 
cylinder head. 
 
The valve seat includes several machined surfaces, as shown in Figure 32. 
The sealing surface is cut to nearly the same angle as the valve face. To 
make a tighter seal and prevent build-up of carbon deposits, two other 
surfaces are machined: a wide-angle upper clearance and a narrow-angle 
lower clearance. 
 
 
 
 
 
 
 
 
 
 
 
 
When the upper and lower clearances are correct, and the valve seat has 
the proper width, the valve face will close tightly against the seat. The 
machining of these surfaces must be very accurate. If valve seat inserts 
are used, the surfaces are machined after the insert has been pressed 
into the valve port. The seat must also be installed properly so that 
efficient heat transfer can take place.