Mazda Training manual — part 245

4 – VALVE TRAIN

65

Piston Engine Fundamentals

TC010-05-01S

VALVE ADJUSTMENT

When a valve closes, there must be a small amount of clearance between
the end of the valve stem and the rocker arm. This clearance is called by
various names such as valve lash, tappet clearance, or valve clearance.

Engines that have solid lifters require periodic valve adjustments to make
sure the clearance is correct. The proper valve clearance eliminates noise
and excess friction.

In some engines, the clearance is adjusted by using shims, which are small
metal disks that come in various thicknesses. Figure 46 shows a tappet
and adjustment shim. By changing the thickness of the shim, the technician
adjusts the valve clearance.

Valve Clearance

The purpose of valve clearance is to make sure that the valve can close
tightly against its seat during compression and combustion. With no
clearance, the valve can be held slightly off its seat, causing compression
pressure to leak past the valve. When the valve remains partially open,
the valve head does not transfer heat to the seat, and combustion gases
will gradually burn the valve away.

On the other hand, if there is too much clearance, the valve train parts can
hammer against each other, causing a ticking noise often called ticking
valves.

FIGURE 46. In
some engines, a
shim is used to
adjust the valve
clearance.

Adjustment
shim

Tappet

4 – VALVE TRAIN

66

Piston Engine Fundamentals

TC010-05-01S

Answers to
Review
Exercise 5

1. face

2. valve

guide

3. heat

4. working

height

5. C

(DOHC)

6. A

(OHV)

7. B

(SOHC)

8. timing

belt,

timing chain

9. B, D

FIGURE 47.
Hydraulic lash
adjusters auto-
matically adjust
valve clearance.

Camshaft

Bucket-type

hydraulic

lash

adjusters

(HLAs)

HYDRAULIC LASH ADJUSTERS (HLA)

On other engines, the valve clearance is adjusted automatically by
hydraulic lash adjusters (HLAs)

. HLAs eliminate the need for manual valve

adjustments.


Description and Operation

The HLA is mounted above the valve stem. A bucket-type HLA is
positioned between the top of the valve stem and the camshaft, as shown
in Figure 47. In this design, the camshaft directly contacts the top of the
HLA. In some engines, a rocker arm-mounted HLA fits between the valve
stem and the rocker arm. (Rocker arm-mounted HLAs are described on
page 68.)

4 – VALVE TRAIN

67

Piston Engine Fundamentals

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Bucket-Type HLA

The bucket-type HLA, shown in Figure 48, has a bucket body that contains
two oil chambers. Movement of oil between these two chambers is
controlled by a check ball and spring.

As the oil moves from one chamber to the other, a spring-controlled
plunger moves up and down, and contacts the top of the valve stem.

FIGURE 48. A
bucket-type HLA
has two oil
chambers, a
check ball, and a
plunger.

Bucket
body
Body

Plunger
spring

Check ball
cage

Check ball

Check ball
spring

Plunger

4 – VALVE TRAIN

68

Piston Engine Fundamentals

TC010-05-01S

FIGURE 49.
Rocker arm-
mounted HLAs
are positioned
between the
valve stem and
the rocker arm.

Rocker
arms

HLAs

Camshafts

Rocker Arm-Mounted HLA

Rocker arm-mounted HLAs operate much like bucket-type HLAs, except
that the top of the HLA contacts the rocker arm, rather than the camshaft.
Figure 49 shows how rocker arm-mounted HLAs are positioned in a DOHC
engine.

















An HLA mounted on a rocker arm does not have a bucket body, but the
check ball, plunger, and body work the same way to maintain a valve
clearance of 0.

Regular Oil Changes

Because the plunger and body of the HLA fit so closely together, regular
oil changes according to the maintenance schedule are very important.
Even small particles of dirt can clog an HLA, resulting in a ticking noise.
Regular oil changes help prevent HLA noise.