Copyright by Goodheart-Willcox Co., Inc.
502 Auto Engine Repair
Figure 23-1. The front end or timing mechanism on today’s
engines can be complex. You must be knowledgeable of proper
repair methods to successfully service modern engines. (Jaguar)
Today’s engines, as you have learned, are much more
complex than the ones used in passenger vehicles just a
few years ago. V-8 engines with two or four overhead cam-
shafts and variable valve timing are now built by several
manufacturers. Long timing chains or timing belts are
needed to smoothly and dependably rotate the camshafts
with the crankshaft. Conventional cam-in-block engines
with short timing chains or timing gears are also manu-
factured. This makes the service of an engine front end
more challenging than ever before. An engine’s front end
includes the timing mechanism, front cover, and related
seals and parts. See Figure 23-1.
This chapter summarizes the most critical informa-
tion concerning the service and repair of timing chains,
timing belts, and timing gears. Timing mechanism service
is critical. If a mistake is made, severe engine damage or
performance problems may result. Improper crankshaft-
to-camshaft timing can cause the engine pistons to slam
into the opened valves. Valves can be bent or snapped off
at their heads. Pistons and cylinder heads can also suffer
severe damage when a valve is broken off by incorrect
valve timing.
Note: Earlier chapters covered information
useful in understanding the material in this
chapter. For instance, Chapter 20 explained
engine disassembly, including front end disassem-
bly. Chapter 11 explained the types and construction
of camshaft drives.
In-Vehicle Timing Mechanism
Service
Often, a timing chain, belt, or gears can be serviced
without removing the engine from the vehicle. Usually, the
parts on the front of the engine and the timing cover can be
removed without complete engine disassembly. In some
OHC engines, the camshaft(s) must be secured before
timing mechanism removal. If this is not done on these
engines, the camshaft(s) will slip out of time and can cause
engine damage. Refer to the service manual for specific
directions as needed.
Timing Chain Service
Timing chains transfer rotation from the crankshaft to
the camshaft. In some designs, they also power other devic-
es. Timing chains can be found in both overhead valve and
overhead camshaft engines. OHC timing chains often have
a complex set of guides, chain adjusters, and idler gears to
maintain chain tension and prevent chain slap.
Timing Chain Problems
Timing chains and their drive sprockets suffer from
wear after prolonged service. The chain links can wear and
stretch. The sprocket teeth can also wear down. This can
cause excess chain slack or the chain can be loose on its
sprockets. A badly worn timing chain can have so much
slack that it hits the side of the front cover and produces a
loud noise.
Timing chain wear and slack can also allow the cam-
shaft to go out of time with the crankshaft. As a result, valve
timing can be thrown off. The valves can open too late
during the four-stroke cycle. This can reduce compression,
power, fuel economy, and exhaust cleanliness.
Inspecting a Timing Chain
You should inspect a timing chain before removal.
Figure 23-2 shows one method of checking OHC timing
chain wear. Use a wrench to rotate the camshaft sprocket
back and forth. Measure the distance the camshaft sprocket
turns before the crankshaft begins to turn. Compare your
measurement to specifications.
Another method to check wear is to measure the
maximum deflection of the chain. This is also shown in
Figure 23-2. Generally, if you can deflect one side of a
timing chain more than about 1/2″ (13 mm), replace the
chain and sprockets.
You can also check timing chain wear on an OHV
engine without removing the timing cover. First, remove
the valve cover. Then, place a large wrench on the
crankshaft snout bolt. Rotate the crankshaft one way and
then the other while watching the valve train. The valves
should operate with little crankshaft movement. If you
can turn the crankshaft more than about 5° to 10° without
valve motion, the timing chain is worn and loose.
If the timing chain is worn more than specifications,
it should be replaced. Many manufacturers recommend
replacement of the chain and both sprockets as a set.