Fan Belt Replacement On Your Air-Cooled 911

One of the most important tasks in maintaining your 911 is the proper replacement and tensioning of the fan belt. If the belt and pulley are not properly attached, it can come loose and stop the cooling action of the fan on the motor. If you don’t happen to notice the temperature gauge or the alternator lamp illuminate, then you may overheat and destroy your engine! Needless to say, the fan belt is very important.

Ironically, it’s also one of the most misunderstood items on the car, and is often improperly tightened. The system works using a set of shims that regulate the thickness of the virtual pulley created by the pulley half and the fan. As you add more shims in-between the pulley half and the fan, the tension on the fan belt decreases because the pulley width is larger, and the belt rides lower in the valley. In an opposite manner, when the shims are removed from in-between the pulley half and the fan, it pinches the belt tighter. As a result, the belt will ride higher and further towards the outside of the pulley, increasing the tension on the entire belt. The system is similar to the one used on the 356, and has worked well for many, many years.

The key to installing the belt properly is to follow a set procedure that will guarantee the proper seating of the pulley and the proper tensioning of the belt. Start by installing five shims on the inside of the pulley and one on the outside. It is very important to note that the pulley must always have a total of six shims on it, inside and out at all times. If you donÂ’t place the extra shims on the outside of the pulley, the nut will bottom out, and the belt won’t tighten properly. If you are missing some shims, get some and put them in right away.

Make sure that you tighten the pulley completely. To achieve this, tighten as much as you can, then turn the starter over a bit, and then retighten. This will give the belt a chance to become unpinched from the pulley. You should be able to feel the point at which all the shims are tight against the fan and the outer pulley half: you won’t be able to tighten any more no matter how hard you try. The final factory torque specification for this nut is 40 Nm.

The tension of the belt should be tight enough so that the belt can be deflected with your finger about 10-15mm at the half-way point between the two pulleys. If the belt is too tight, move some shims to the inside. If it’s too loose, remove some from the inside and move them to the outside. At all times keep the total number of shims inside and out at six.

By Wayne Dempsey
Reprinted from
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