The Air-Cooled Thermostat Explained

by Will Wood

"I don't have a thermostat and I am wondering if I need one..."

Any engine when it warms up expands in size-- watercooled or aircooled, it doesn't matter. It's also important that an engine warms up to operating temperature as quickly as possible. Why? A cold engine is an engine with more friction and the faster the warm up, the less sludge that builds up in the oil from combustion. There's also another reason in an aircooled VW - Expansion rates. Aluminum expands faster than steel or iron, so your heads expand faster than your cylinders. The thermostat allows both to expand evenly (or as near evenly as the VW engineers could account for).

Also, when the thermostat is closed, i.e., not as much air pushing over the engine allowing it to warm up, you use less horsepower. Yes, it takes horses to push the cooling fan, when the thermostat is closed, it takes less power. So, in colder climates you may find the thermostat is mostly or fully closed during operation. When the engine warms up, it opens and the flaps open up.

One area that VW didn't do a very good job on is the Type IV cable assy. This has to be checked, and is the weak link in the Type IV system. However, a properly operating thermostat and flap system Type I, II, III or IV will insure faster warm ups, reduced wear and reduced sludge build-up.

Remember the classic experiment describing air pressure that used an old paint can?: Fill the can with water, heat it to produce steam, then seal the can up. Now, cool the can quickly (or slowly)...and what happens? The can crushes due to low air pressure (the steam condensing inside the can) and high air pressure outside. The same thing happens with the VW thermostat. They're filled, and heated and brazed expanded, which forces the normal air pressure out. The gas produced from the liquid (not water, but something with a lower boiling point) pushes the air out. The brazing seals the chamber and then when cool, the thermostat collapses. When you re-introduce air (i.e., puncture), the thermostat expands because the metal is under force to be collapsed.

As for thermostats failing constantly, I haven't had but one fail in nearly 20 years of tinkering with these things. It was my fault as well as I didn't get the expansion bracket properly aligned and the thing bowed out. It still didn't burst, just expanded way beyond the norm. That's what the expansion bracket is for. When installing it keep it vertical....

As St. Berg once stated: "If it cost $5 to put a part on and it wasn't Necessary, VW could have saved over $50,000/day by leaving it off."

Back to Library Back to Cooling