Fan Belt and
Pulley Replacement

Difficulty Level 2

Difficulty scale:
Adding air to your tires is level one
Rebuilding a 911 Motor is level ten

     Unfortunately, the only real warning of a belt failure is the alternator light indicating that you have a problem.  I was driving my 2.7 914-6 on a winding road in bright sunlight, coming back from a short trip when I noticed that my alternator light was on.  The sun was shining on the dash, so I barely noticed it.  Since I have had many alternator problems in the past, I simply assumed that my alternator was going bad.   About three minutes later, I saw that my temperature gauge was almost in the red.   At this point, I knew that my fan belt had broken, and I had to stop real soon.   I limped into a gas station and shut off the car.  Smoke was billowing from my engine (left over oil that had spilled on the engine block I assume), and when I lifted the engine grille, I could hear the oil 'bubbling' inside the engine.  Needless to say, I'm glad that I noticed the alternator light in time.  This incident enlightened me to the importance of this seemingly simple part.

     Needless to say, if your fan belt or pully fails, then your engine could be in trouble.  The key to keeping your motor running cool is preventative maintenance in this case.  Proper installation of both the fan and the pulley will guarantee that your car will be running cool for thousands of miles.

     You should frequently inspect your fan belt for damage or wear from the years of previous service.  The belt often shows signs of cracking or drying out after years of service in the car.  You should check the fan belt every time that you change the oil, and replace it if you see anything that looks wrong.   The tension of the fan belt is adjusted by the use of shims around the pulley.   Adding more shims on the shaft causes the belt to ride lower on the pulley shaft.   Removing the shims decreases the diameter of the 'V' groove, and causes the belt to ride higher, and thus tighter. 

Figure 2:
Destroyed 911 Fan Pulley

Figure 3:
New Pulley and Shims

Figure 4:
New Replacement Shims

Figure 5:
911 Fan Belt Pulley Parts Exploded View

     Once you have the old pulley, belt and shims off, take a close look at the pulley.  Figure 2 shows a pulley that was damaged, probably due to overtightening.  These pulleys are used primarily in smog or AC motors where you need to run another device off of the main driveshaft.  In general, the earlier pulleys are stronger in construction.   The pulley actually broke in its center ribs and then became very loose on the shaft.  Figure 3 shows an earlier type pulley with larger holes that are less prone to break.  In addition, you want to make sure that you use the proper sized shims.  I found out that on my car, someone had used shims that were too thin.  Perhaps that added to the problem, and caused the pulley to fail.  Figure 4 shows the proper sized shims for a 1974 911 motor.

     Figure 5 shows the correct assembly order of the shims, pulley and belt on the shaft.  Although the factory recommends storing the extra shims on the outside of the pulley (as shown in the Figure) I don't think that this is a very wise idea.  I think that the placement of the shims on the outer part of the shaft may have contributed to the failure of my original pulley.  As shown previously in Figure 2, the shims are very worn, indicating that they were possibly spinning on the shaft.   Note that these were the extra shims on the outside that showed this characteristic wear.

     After publishing this article, Charlie Swanson made the following observations.

"The extra shims must be installed as per the factory recommendation or the washer will bottom on the shaft and won't clamp the pulley to fan. When this condition exists the pulley will be loose and wear into the shaft. It happened to me (I put the 2 extra shims in the glove box) and nearly destroyed the shaft. If you don't believe me, ask Bruce Anderson."

     It would seem that the factory designed the shaft to account for the shims.  Be careful to make sure that the pulley is tight if you do decide to not put all the shims on the shaft.

    You begin the installation process by placing 5 shims on the shaft in-between the pulley halves.  Using the screwdriver or the pulley holding tool, tighten down the belt until it is reasonably tight.  At this point, the pulley is probably compressing the belt against the fan housing, and is not completely seated.   To seat the belt, you need to rotate the engine slightly.  You can accomplish this by unplugging the CD box (the small box that makes the high pitched sound, located on the left side wall of the engine compartment) and turning the engine over a few times with the starter.  This should cause the belt to ride up, and prevent it from being pinched.  Retighten the pulley after you turn the engine over.  Repeat this process until the pulley nut is cannot be turned any more.   At this point, the pulley should be seated up against the fan.  You should be able to determine when this happens by the different pressure when tightening.

     Once the pulley is tight against the fan, check the belt tension.  The belt is at the correct tension when it can be deflected about one-half   to three-quarters of an inch under light thumb pressure.  If the tension is a little too loose, then remove one of the shims, and start again.  Make sure that you keep turning the engine over after you tighten the pulley.  This is the key to making sure that everything is tightened correctly.

     Well, that's really all there is to it.  If everything is done correctly, then you should have no problems.  It's a relatively simple task, but the results of potentially messing it up are very great.  If you have any questions about this particular tech article, please feel free to ask.  Remember that your continued support of Pelican Parts helps keep this web site alive and growing.  Please consider us for all your 911 parts purchases.