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

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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.

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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.