Neglect Could Lead To Expensive Engine Repair
Be it an Acura or a Yugo, a
failed timing belt is not a pretty sight.
The engine, of course, will stop
if the timing belt breaks, gets stripped of enough teeth or
But the real danger lies not in
the inconvenience of being stranded on the road. It is in the
real possibility of severe engine damage, warn engineers from
The Gates Rubber Company.
Automobile engines can be
classified as either free-running or interference, depending on
what occurs if the piston-to-valve synchronization is lost due
to a failed timing belt.
In the free-running
(free-wheeling) engine with the crankshaft still moving, there
is still enough clearance between the valve and piston, even if
the cam stops with a valve fully open.
However, an interference engine
may allow the piston and the valve to collide. The least damage
could be a bent valve. The most damage may be a hole in the
piston, or damage to the head, or gouged cylinder heads, or
worse yet, a completely ruined engine.
Most domestic cars are
free-running. Several notable exceptions are some models of the
Chevy Spectrum and Chevette, Plymouth Colt, and the Pontiac
LeMans and Sunbird. (The majority of these models have
Most import cars, however, have
higher compression engines, and are most likely to be the
interference type. More than a dozen manufacturers, from Acura
to Yugo, have produced this type of engine.
Also, all diesel engines are
interference. A compression ratio of 20:1 doesn't leave much
combustion chamber space to spare.
Rather than risk an expensive
engine repair, Gates suggests car owners heed the advice of the
auto manufacturers and most independent repair technicians.
The auto makers say that timing belts should be replaced at the
time or mileage interval specified in their owner's manuals,
regardless of the physical appearance of the belt.
Independent technicians agree with the OE's, but to be safe,
they also recommended that the belt be inspected between 30,000
and 50,000 miles. This inspection process can vary
depending on the engine placement, and the number and location
of engine accessories. The most practical time to check
the belt is during a tune-up, while the spark plugs are out and
there is no engine compression. A simple inspection may
involve rotating the drive and feeling for slack or play in the
belt. This may indicate a worn or stretched belt, or a loose
tensioner. On some engines, the belt guard can be partially
pulled away for a quick visual inspection.
The most thorough inspection,
however, involves removing the belt cover to check every tooth
and land (the flat area between the teeth). Care should be taken
when handling the belt, though. Do not twist the belt more than
90o, or the tensile cords could be damaged. Look carefully
at the sides and top of the timing belt. Watch for worn or
frayed fabric tooth facing, glass fibers protruding through the
belt, tooth cracks, scratches or grooves on the back of the
belt, or oil on the belt or sprockets.
A timing belt can have hidden
damage. Gates explains that sometimes the tensile cords can fail
due to an engine malfunction resulting in a shock load to the
belt. Cords can also be damaged by a foreign object as small as
a pebble coming between the belt and sprockets.
Also, the integrity of the belt
environment is especially important. If dirt, grease and oil are
permitted within the timing belt cover, the chances for belt
failure are greatly increased.
Especially if it's a
valve-crunching interference engine, it shouldn't be too hard to
convince anyone to check the belt, and replace it at the first
signs of fatigue.
If you wait for the belt to
break, you'll be sadder and wiser--but poorer.
Contribution by The Gates Rubber Company.