Tight Engine Timing Belts Can Cause Havoc - Engine Builder Magazine

Tight Engine Timing Belts Can Cause Havoc

If one day it sounded like a cat was being tortured under the hood, you
got out your trusty wrenches, loosened the generator mounting bolts,
and tore up your knuckles as you used a pry bar, pipe or 2×4 to pry
away until it stoped making noise. You then tightened the bolts again
and went on your merry way. The belts were long enough and had enough
stretch and slop to them that it was nearly impossible to damage

Well, as time went on and more accessories were hung off the front
of the engine, belt tension and deflection actually became a
specification. Why? Because if you put the old gorilla effect on the
belts you could actually cause crankshaft damage.

Now before you think that I fell into the mushroom patch let me
explain what would and did happen quite often. If there was enough
torque put on the front pulley (that at times would hang 6-8 inches or
more off the front of the crankshaft), enough load could be placed up
on the front main bearing in an upward fashion that it could actually
take the clearance out of the bearing  (see Figure 1).  

If you look at the lower portion of Figure 1, you can see that with the
crankshaft lying in the main saddles on centerline you will have
bearing clearance through the entire circumference once in motion. This
is what it should normally be.

The crankshaft on numerous engines would actually break at the #1
rod journal because of the stress load of the belts holding the
crankshaft upward and the combustion power trying to drive it back
down. It is like taking a thin piece of steel and bending it back and
forth until it breaks.  The other failure that would commonly occur
would be scoring on the upper half of the  #1 main bearing. This was
often caused by oil not being able to flow properly from the block to
the bearing (see Figure 2).

I have to be honest – I haven’t heard a lot of reports of this failure
for some time. With the introduction of the serpentine belt and auto
tensioner that type of failure seemed to go away never to be seen
again. Or so I thought. As it turns out, this problem has reared its
ugly head once again. I suspect that much of it has to do with the
current economic climate and a much higher rate of individuals working
on their own more complex engines.

What I am talking about are overhead camshaft applications that now use
timing belts. Many of them are very complex and may even have multiple
belts being driven off the crank (see Figure 3).
However, when the manual says to replace the timing belt at 60,000
miles and the cost to have that belt replaced may exceed $200, some
drivers are deciding to do the work themselves. After all, how hard can
it be? A belt is a belt isn’t it?

Well, no. Many of the engines in current production have hydraulic
or spring loaded belt tensioners, but many still have manual tensioners
that require either special tools and/or removal/installation
procedures that are not always followed. Why? Because the details on
left-handed threads or multi-part tensioners may not be available to
the average DIYer. So the old school of tighter is better kicks in and
low and behold the front main bearing soon looks like the one in Figure

Figure 4 shows a typical illustration that you may see
for a manual tensioner installation. As you can see a special tool is
required and an illustration is given of belt deflection measurement.
There is often a specific torque value or deflection at a certain level
of pounds-pull given. Of course there is that other minor detail of
having the engine timed correctly and if the tensioner is not set right
you may end up having the valves crash into the pistons.  

The bottom line here is that premature engine failures are on the rise
due to overtightened belts. When timing belts are being installed by
someone who does not have the correct information, the experience or
expertise to do it properly, the front main bearing shown in Figure 2
may start showing up at your door along with the question of “Why did
my engine fail?” 

The best way to avoid this question may be to install the belt yourself
if asked, or at least provide the information so that there is a better
chance that a premature failure does not occur.

Roy Berndt has decades of machine shop experience.
He is the Program Manager for PROFormance
Powertrain Products, a PER in Springfield, MO.
figure 1 at top, the shaded bar denotes a crankshaft being pulled upward on the front left while the red line indicates the center line of the main bearing housing bores. at the bottom, the shaded area denotes a crankshaft under normal circumstances lying straight with the center line of the main bearing housing bore.figure 2 note the upper bearing on the right that is housed in the block being worn dramatically worse than the lower bearing that should be the load side. this failure was due to excessive timing belt pressure.figure 3 note the complexity and amount of belts and tensioners being used for the early b234f volvo engine with manual tensioner.Figure 4 This illustration shows the use of special tools and deflection of a typical manual timing belt tension illustration.

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