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If one day it sounded like a cat was being tortured under the hood, yougot 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 pryaway until it stoped making noise. You then tightened the bolts againand went on your merry way. The belts were long enough and had enoughstretch and slop to them that it was nearly impossible to damageanything.

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

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

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

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

I have to be honest – I haven’t heard a lot of reports of this failurefor some time. With the introduction of the serpentine belt and autotensioner that type of failure seemed to go away never to be seenagain. Or so I thought. As it turns out, this problem has reared itsugly head once again. I suspect that much of it has to do with thecurrent economic climate and a much higher rate of individuals workingon their own more complex engines.

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

Well, no. Many of the engines in current production have hydraulicor spring loaded belt tensioners, but many still have manual tensionersthat require either special tools and/or removal/installationprocedures that are not always followed. Why? Because the details onleft-handed threads or multi-part tensioners may not be available tothe average DIYer. So the old school of tighter is better kicks in andlow and behold the front main bearing soon looks like the one in Figure2.

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

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

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

Roy Berndt has decades of machine shop experience.He is the Program Manager for PROFormancePowertrain 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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Roy Berndt
Roy Berndt has decades of machine shop experience. He is the Program Manager for PROFormance Powertrain Products, a PER in Springfield, MO. You can reach Roy at [email protected]