Mazda Training manual - part 241

 

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Mazda Training manual - part 241

 

 

 
4 – VALVE TRAIN 

 

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VALVE GUIDES

 

 
Some valve guides are machined directly into the cylinder head casting. 
Other guides are soft alloy inserts that are pressed into the head, as 
shown in Figure 33. In some cases, the valve guide is reamed (drilled out) 
for a close fit to the valve stem after the guide has been installed. 
 
The valve guide fits very closely around the valve stem, with just enough 
room for lubricant and free movement of the stem. When the guide wears 
out, it can sometimes be reamed out so that a valve with an oversized 
stem can be installed. Insert-type guides are replaceable. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 
 
 

FIGURE 33. Valve 
guides hold the 
valve stem so the 
valve fits 
squarely on its 
seat.

 

 

Cylinder 
head

 

Valve stem

 

Valve guide

 

Valve seal

 

 

 
4 – VALVE TRAIN 

 

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FIGURE 34. 
Springs are used 
to close the 
valves 
 

Valve 

Keepers 

Upper spring 
seat 

Spring 

Lower spring 
seat 

Valve seal 

Valve guide

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

VALVE SPRINGS

 

 
Springs are used to close the valves firmly against their seats. A typical 
valve spring assembly is shown in Figure 34. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The valve is installed in the cylinder head, with the spring and upper 
spring seat over the stem. Then the spring is squeezed, and the keepers 
are fitted into the groove in the valve stem. The keepers form a collar 
around the valve stem, and hold the spring in place. On some engines, 
the keepers allow the exhaust valve to rotate in the valve guide, resulting 
in equal heat distribution, more even wear, and ‘self-cleaning’ of the 
valve-to-seat surface. 
 

 
4 – VALVE TRAIN 

 

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Most overhead valves also use an oil seal to prevent oil from being drawn through the valve 
guide and into the combustion chamber. The seal may be a cup type that fits over the end 
of the valve guide, or it may be installed directly on the valve stem. 

 
 
 

Construction

 

 

Valve springs are made of high quality steel to exact specifications for spring force and 
squareness. The ends are ground flat, and squareness is checked carefully so the spring 
will not tilt the valve. 

 
 
 

Spring Tension

 

 

The valve spring requires very high tension to close the valve completely. As the engine 
speed increases, higher force is needed. If the spring is weak, the valve will tend to float, or 
fail to seat at high speed. This condition causes lost power and burned valves. 

 

On the other hand, the spring tension must be limited because of wear to the cam lobe, 
lifters, and other valve train parts. Since spring tension can be lost over time, used springs 
are checked on testers to make sure the tension is correct at the normal working height of 
the spring. 

 
 
 

Working Height

 

 

Working height 

is the length of the installed spring between the retainer and the spring pad 

on the cylinder head when the valve is fully closed. When valve faces and/or seats are 
refinished, the working height increases slightly, resulting in less spring tension. To make 
up for this loss, the technician installs one or two shims under the spring to compress it to 
the correct height. 

 

 
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FIGURE 35. In an 
OHV design, 
lifters and push 
rods transfer the 
action of the 
camshaft to the 
rocker arms. 
 

Rocker arm 

Valve 

Pushrod 

Lifter

 

Cam

 

Camshaft

 

 
 
 
 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

OVERHEAD VALVE TRAIN

 

 
An overhead valve (OHV) train design means that the valves are 
mounted in the cylinder head at the top of the combustion chamber. 
OHV engines have the camshaft installed below the valves, in the 
cylinder block. Figure 35 shows the major parts of the overhead valve 
train. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

As the camshaft turns, the cam lobe moves against the lifter (sometimes 
called a tappet or cam follower). The lifter pushes up on the pushrod, 
which contacts the rocker arm. The valve end of the rocker arm pushes 
down on the valve and opens it. As the cam lobe moves past the lifter, the 
valve spring pushes against the rocker arm, and the pushrod moves down, 
allowing the valve to close. 

 
 
 

 

 

 

 

 

 

 

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