chap27 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 27, the reader should
be able to:

•
•
•
•
•

Prepare for ASE Engine Repair (A1)
certification test content area “C” (Engine
Block Diagnosis and Repair).
Describe how the camshaft and valve train
function.
Discuss valve train noise and its causes.
Explain how to degree a camshaft.
Explain how a hydraulic lifter works.

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KEY TERMS:
bucket • cam follower • camshaft bearings • contour
duration of the camshaft
finger follower • flat-link type • freewheeling
hydraulic valve lifter • intake centerline • intake lobe
centerline method
minutes • Morse • roller chain type
silent chain type • solid valve lifter • straight up
tappets • valve lash
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The cam is driven by timing gears, chains, or belts
located at the front of the engine.
The gear or sprocket on the camshaft has twice as
many teeth, or notches, as the one on the crankshaft.
This results in two crankshaft turns for each turn of the
camshaft. The camshaft turns at one-half the crankshaft
speed in all four-stroke-cycle engines.

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CAMSHAFT FUNCTION
The camshaft’s function is to operate the valve train. Cam shape 
or contour is the major factor in determining the operating 
characteristics of the engine. 
The lobes on the camshaft open the valves against the force of the 
valve springs. The camshaft lobe changes rotary motion 
(camshaft) to linear motion (valves).
The camshaft may also operate the following:
Mechanical fuel pump
Oil pump
Distributor
5

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Cam lobe shape has more control over engine performance 
characteristics than does any other single engine part.
Engines identical in every way except cam lobe shape may have 
completely different operating characteristics and performance. 

Figure 27–1 This high-performance camshaft has a lobe that opens the valve quickly and keeps
it open for a long time.
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Figure 27–2 In many engines, the camshaft drives the distributor and the oil pump through a
shaft from the end of the distributor.

7

Continued
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Figure 27–3 The fuel pump plunger rides on the camshaft eccentric.

8

Continued
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By James D. Halderman

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CAMSHAFT LOCATION
Pushrod engines have the cam located in the block. They are 
smaller and lighter than overhead cam engines. The camshaft is 
supported in the block by camshaft bearings and driven by the 
crankshaft with a gear or sprocket and chain drive. 

Figure 27–4 Cutaway of a
Chevrolet V-8 showing the
valve train components.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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CAMSHAFT PROBLEM DIAGNOSIS
A partially worn lobe on the camshaft is often difficult to 
diagnose. Sometimes a valve “tick tick tick” noise is heard if the 
cam lobe is worn. 

The noise can be intermittent, which makes it harder to determine 
cause. If the engine has an overhead camshaft (OHC), it is usually  
easy to remove the cam cover and make an inspection cam lobes 
and valve train.

In an over­head valve (OHV) engine, the camshaft is in the block, 
where easy visual inspection is not possible. 

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The Rotating Pushrod Test
To quickly and easily test whether or not the camshaft is okay, observe if
the pushrods are rotating when the engine is running. This test will work
on any overhead valve pushrod engine that uses a flat-bottom lifter.
Due to the slight angle on the cam lobe and lifter offset, the lifter (and
pushrod) should rotate whenever the engine is running.

To check, remove the rocker
arm cover and observe the
pushrods with engine running.
If one or more is not rotating,
this camshaft and/or the lifter
for that valve is worn and
needs to be replaced.
Figure 27–5a Here is what can happen if a
roller lifter breaks loose from its retainer.
The customer complained of “a little noise
from the engine.”
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Figure 27–5b All engines equipped with roller lifters have some type of retainer for keeping the
lifters from rotating.

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CAMSHAFT REMOVAL
If the engine has an overhead valve design, the camshaft is usually 
located in the block above the crankshaft. The timing chain and 
gears (if so equipped) should be removed after the timing chain 

cover is removed.
Loosen rocker arms (or rocker arm shaft) and remove the pushrods. 
Remove or lift up the lifters before carefully removing the 
camshaft.

NOTE: Be sure to keep the pushrods and rocker arms together if they are
to be reused.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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The Tube Trick

Valve lifters are often difficult to remove because the ends of the lifters
become mushroomed (enlarged) where they have contacted the camshaft.
Varnish buildup can also prevent lifters from being removed. Try this:
Step #1 Raise the lifters upward as far away from the camshaft as
possible.
Step #2 Slide in a thin plastic or cardboard tube with slots in place of the
camshaft.
Step #3 Push the lifters downward into the tube. Use a long magnet to
retrieve the lifters from the end of the tube.
This trick will work on almost every engine that has the camshaft in the
block. If the tube is made from plastic, it has to be thin plastic to allow it to
flex slightly. The length of the lifters is greater than the diameter of the cam
bearings. Therefore, the lifter has to be pushed downward into the tube
slightly to allow the lifter room to fall over into the tube.
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CAMSHAFT DRIVES
The crankshaft gear or sprocket that drives the camshaft is usually 
made of sintered iron. When gears are used, the camshaft gear
teeth must be made from a soft 
material to reduce noise. Usually,
the whole gear is made of
aluminum or fiber.
Figure 27–6 The larger camshaft gear is
usually made from fiber and given a helical
cut to help reduce noise. By making the
camshaft gear twice as large as the
crankshaft gear, the camshaft rotates one
revolution for every two of the crankshaft.

When a chain and sprocket are used, 
the sprocket may be made of iron or 
have an aluminum hub with nylon 
teeth for noise reduction.
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Two types of timing chains are used:
1. The Silent chain type (known as a flat­link type, or Morse 
type for its original manufacturer). This type operates quietly 
but tends to stretch with use. 

Figure 27–7 A replacement silent chain
and sprockets. The original camshaft
sprocket was aluminum with nylon teeth
to help control noise. This replacement
set will not be noticeably louder than the
original and should give the owner many
thousands of miles of useful service.

Continued
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NOTE: When the timing chain stretches, valve timing will be retarded 

and the engine will lack low­speed power. In some instances, the chain can 
wear through the timing­chain cover and create an oil leak.

Figure 27–8 The industry standard for when
to replace a timing chain and gears is when
1/2 inch (13 millimeters) or more of slack is
measured in the chain. However, it is best to
replace the timing chain and gear anytime the
camshaft is replaced or the engine is

disassembled for repair or overhaul.

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2. The Roller chain type. This type is noisier but operates with less 
friction and stretches less than the silent type of chain.

Figure 27–9 A replacement high-performance double roller chain. Even though a bit noisier than
a flat-link chain, a roller chain does not stretch as much and will therefore be able to maintain
accurate valve timing for a long time.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

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Some four­cam engines use a two­stage camshaft drive system: 
Primary: from crankshaft to camshaft 
Secondary: from one camshaft to another

Figure 27–10
Typical dual overhead camshaft V-type
engine that uses one primary timing
chain and two secondary chains.
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The Noisy Camshaft
The owner of an overhead cam four-cylinder engine complained of a noisy
engine. After taking the vehicle to several technicians and getting high
estimates to replace the camshaft and followers, the owner tried to find a
less expensive solution. Finally, another tech replaced the serpentine drive
belt on the front of the engine and “cured” the “camshaft” noise for a
fraction of the previous estimates.
Remember, accessory drive belts can often make noises similar to valve or
bad-bearing types of noises. Many engines have been disassembled
and/or overhauled because of a noise that was later determined to be from
one of the following:

•
•
•

Loose or defective drive belt(s)
Loose torque converter-to-flex plate (drive plate) bolts (nuts)
Defective mechanical fuel pump

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CAMSHAFT BELT DRIVES
Many overhead camshaft engines use a timing belt rather than a 
chain. The belt is generally quieter, but it requires replacement, 
usually every 60,000 miles (100,000 km). Unless the engine is 
freewheeling, the piston can hit the valves if the belt breaks. 

Figure 27–11 Broken timing belt. Also notice
the missing teeth. This belt broke at 88,000
miles because the owner failed to replace it
at the recommended interval of 60,000 miles.

Continued
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Figure 27–12 This timing belt broke because an oil leak from one of the camshaft seals caused
oil to get into and weaken the belt. Most experts recommend replacing all engine seals in the
front of the engine anytime a timing belt is replaced. If the timing belt travels over the water
pump, the water pump should also be replaced as a precaution.

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Figure 27–13 Many engines are of the interference design. If the timing belt (or chain) breaks, the

piston still moves up and down in the cylinder while the valves remain stationary. With a freewheeling design, nothing is damaged, but in an interference engine, the valves are often bent.
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Best to Warn the Customer
A technician replaced a timing chain and gears on a Chevrolet V-8. The
repair was accomplished correctly, yet, after starting, the engine burned an
excessive amount of oil. Before the timing chain replacement, oil
consumption was minimal.
The replacement timing chain restored proper operation of the engine and
increased engine vacuum. Increased vacuum can draw oil from the
crankcase past worn piston rings and through worn valve guides during

the intake stroke. Similar increased oil consumption problems occur if a
valve regrind is performed on a high-mileage engine with worn piston rings
and/or cylinders.
To satisfy the owner of the vehicle, the technician had to disassemble and
refinish the cylinders and replace the piston rings. Therefore, all
technicians should warn customers that increased oil usage may result
from almost any repair to a high-mileage engine.

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ROCKER ARMS

Rocker arms reverse upward movement of the pushrod to produce 
a downward movement on the tip of the valve. They reduce travel 
cam follower, lifters and pushrod while maintaining valve lift. 

This is using a rocker arm
ratio of approximately 1.5:1.
It allows the camshaft to be 
small, so the engine can be 
smaller, and results in lower 
lobe­to­lifter rubbing speeds.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 27–14 A 1.5:1 ratio rocker arm means
the dimension A is 1.5 times the length of B.
If the pushrod is moved up 0.400 inch by the
camshaft lobe, the valve will be pushed down
(opened) 0.400 inch 1.5, or 0.600 inch.

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