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Volkswagen Fan Belt

The fan belt is responsible for spinning the alternator/generator pulley which charges the electrical system and turn the fan in the fan shroud which cools the engine.  If your belt breaks or it thrown, your generator light should come on, notifying you that something is wrong.  If you decide to continue driving, your engine can overheat and damage can ensue.

Belt Adjustment

Because the fan must be properly positioned within the fan shroud for optimal cooling, the VW design doesn't allow for alternator position adjustment for belt tensioning as you would with most cars without a self tensioning belt system.  VW instead developed a shim system that allows you to change the diameter of the alternator/generator pulley to allow for proper belt tension. 

There should be 8-10 shims with the two halves of the pulley on the alternator/generator.  The more shims you have between the pulley halves, the less the overall diameter will be on the alternator pulley.  Shims not used should be placed outside of the pulley for storage.

Tension should be great enough to allow you to turn over the engine by turning the alternator pulley.  Add or remove enough shims until you are just able to turn over the engine.  The play in the middle of the belt should be around 1/2 inch.

Belt Size

The fan belt used by VW has evolved over the years.  They used the 9.5 x 900mm V belt for their 6-volt engines.  The Continental 9.5 x 905mm V belt was later used on the newer 12v generator engines.  Continental later standardized their belts to a 10 x 900mm (111-905-137B) and 10 x 905mm (OEM 111-903-137D) belt and VW came out with new OEM numbers to reflect this change.

Earlier bugs had smaller belt which was sufficient for the demands of their low wattage generators and smaller fans of their time.  The 36hp engine had a fan with 24 blades and a 160 watt generator. The 40hp and 1300/1500/1600cc single-port engines had fans with 28 blades which increases the demand on the fan belt to a small degree.  Plus during that time, the generator output rose from 160 watts to 360 watts.  As a result, the 1500/1600cc single-port engines had a 5 hp load on the fan belt, whereas the earlier engines only had 4hp.  Later, in 1971, the introduction of the doghouse cooler also meant a 5mm wider (10% increase) fan which brought the load on the belt to around 5.5hp.  In 1973, the addition of the 220 watt alternator brought the belt load to a whopping 6hp.  And with signs of early failures from the thin VW belts, it was time to upgrade the belt size, so the 1973 alternator also got a much thicker heavier duty 11.3 x 912 mm v-belt.

In regards to what belt will work for an engine with an alternator or generator - thicker is better.  For the most part, you can run the older thinner belts in a newer engine with an alternator and likewise can run the thicker newer belt in an older engine with a generator.  You will just have to add or subtract shims as needed.  Properly fitted thicker quality belts should last longer than thinner belts in any VW.

Also note that belts from different manufactures also vary to some degree in true size and different combinations of pulleys and belts can have a surprising impact on belt tightness.  The stock pulleys for the generators and alternators are the same, but these pulleys are often replaced with aftermarket ones by previous owners, so you may have to go with a longer or shorter belt to get the perfect fit.  Because of this, you may need to experiment with several belts, remember which one works for your auto and remember to buy a spare.

Stock VW 1600 DP with Alternator Belt:

The US Gates 7355 v-belt is supposed to fit:

In case you need to convert inches to and from mm:

Loose Belt

If your belt is too loose with all the shims removed, you may have a one off pulley or worn alternator bearing.  A slightly smaller than stock belt is one potential fix.  The Gates 7350 (13/32" x 35 5/8") should do the trick.

Tight Belt

If your stock sized belt is too tight, the the Gates 7360 at 36 inches (914.4mm) should work for you.

Continental 9.5 x 905mm for vintage VW diehards

12V Generator Belt

Stock VW 1600 with 12V Generator used a thinner belt (but can still use a thicker one too):

If this is too loose, you can try a 10 x 900mm belt used on the 6V VWs (OEM 111-905-137B)

6 Inch Power Pulley

Power Pulley Belt

For those with a Power Pulley, you will need a smaller belt.

Ref:

Air Flow Curves for the VW cooling fan

Smaller Generator Pulleys

The Porsche 356 generator pulley is smaller and allows you to move your fan faster at lower RPMs.  Some use it to compensate for a smaller crank pulley when installing dry sump pumps and others use it to increase cooling.  Care should be taken not to over tighten the belt as these can easily fail from over fatigue.  This should also make finding a properly sized belt for your application more exciting.

For VW pulleys, the 12 volt Generator/Alternator Pulley (stamped steel) was also used on the large diameter 6 Volt Generator (105mm OD). The very expensive cast-iron backed 6 Volt Pulley was used on the smaller diameter (90mm) 6 Volt Generators (GR11X) back to 1959 and aligns the belt differently than the newer pulleys and therefore is not interchangeable.  An even smaller pulley was used on the 36hp varieties because of their smaller Crank Pulley.

Other Part Numbers

Serpentine Belt

There are a few aftermarket options available.  The CB Performance version can be problematic and the BMD seems like a quality unit that allows for easy belt tension adjustment and decreases or eliminates that possibility of the belt being thrown.

BMD Pulley System

The only potential problem with the serpentine system is that it is possible to over spin the fan.  With a stock setup, once you reach so many RPMs, the V-belt will slip.  With a serpentine system, you may have enough belt friction to turn the fan beyond the maximum RPMs that it is physically able to withstand.

Ref:

BMD Pulleys

CB Performance

off-road.com

 

 

Cogged Pulleys and Belts

Ref:

Bullys Pulleys

 

 

Spare Belt

After a spare tire (with lug wrench and jack), a fan belt is the most important spare item you should have in your VW.  And like a flat tire, you can't drive very far without a belt and shouldn't even try to.

Don't forget your screwdriver and 19mm wrench to change your belt.

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