Is Your Fan Belt Tension Correct? (Aircooled VW 1200–1600 Engines)
Posted by: Kate McCarthy
Category: Engine
Last updated: January 2026
The fan belt on an aircooled Volkswagen is one of the most important components on the entire engine. It doesn’t just drive the generator or alternator — it also drives the cooling fan that keeps your engine alive.
If the belt is too loose, your engine won’t cool properly.
If it’s too tight, you can destroy bearings and wear components prematurely.
Getting the tension right is critical.
Which engines does this apply to?
- Aircooled Volkswagen flat-four engines
- 1200cc, 1300cc, 1500cc and 1600cc
- Beetle, early Bay, Split, Karmann Ghia and Type 3 engines using standard VW pulleys
(If you are running aftermarket pulleys or Porsche-style systems, the setup may differ — always check manufacturer specs.)
What the fan belt actually does
- Drives the generator or alternator
- Drives the engine cooling fan
Without the belt, the engine will overheat very quickly — even at idle.
Correct fan belt deflection
VW specification: 10–15mm (about ½ inch) of deflection
This is measured by pressing down on the belt halfway between the crank pulley and the generator/alternator pulley.
The pulley shim system
The generator/alternator pulley is made of two halves with shims between them:
- More shims between the pulley halves = looser belt
- Fewer shims between the pulley halves = tighter belt
Any unused shims must always be stored under the outer pulley nut.
How to adjust the fan belt
Step 1 – Remove the pulley nut
Use a 21mm spanner or socket to remove the pulley nut.
Step 2 – Count the shims
You should always have 8–10 shims in total.
Step 3 – Adjust tension
- If the belt is too loose: move a shim from between the pulley halves to under the nut.
- If the belt is too tight: move a shim from under the nut to between the pulley halves.
Step 4 – Refit and test
Refit the pulley, tighten the nut, then check deflection again.
Correct belt size
Most 1200–1600 engines use:
9.5 x 905mm
Some alternator setups use:
10 x 905mm
Common mistakes
- Running the belt too tight
- Running the belt too loose
- Missing pulley shims
- Using cheap incorrect belts
Summary
A correctly tensioned fan belt keeps your aircooled engine cool, your charging system working and your bearings healthy. It’s a five-minute job that can save your engine.
