VW Baywindow Bus - Thermostats Explained

by Richard Atwell
(c) Copyright 2005-2022


June 9, 2013 Update:

Awesome Powdercoat is now manufacturing Type 4 thermostats in the USA:

Long engine life is not hard to achieve as long as all the parts are in place and functioning properly. Because the engine is air-cooled, the control of air flow to manage cylinder and oil temperatures is critical to engine life.

Whether or not you live in a hot climate you must have a working thermostat flap system for maximum engine life. Hoover said it best, "Unfortunately, without the thermostat your jugs will wear like a bitch, as will your valve guides; you'll burn more gas, suck a lot of oil and have a hell of a time passing your smog check".

Gene Berg is even more specific. He says missing thermostats increase engine wear 15-17%.

When the thermostat is missing, the engine takes too long to warm up in most climates. If you live where it's 100F all day long every day of the year, you may come to the conclusion that you can leave it off but even where it's that hot it can also drop to typical desert temperatures at night.

If you live in a climate where you can drive all year round in mild temperatures (60-70s) such as the California coast then a thermostat is required. If you live in Canada or parts of the USA where it snows, it's required. If you live in Texas or Florida it's required. No ifs ands or buts. Why?

coolin flaps

The engine expands at it heats and this has to be done in a controlled manner to limit wear. When the engine is kept cold for longer than necessary during warmup, the combustion by-products contaminate the oil and promote acid and sludge formation. If you've ever cleaned out your oil strainer you've probably seen that not everything inside the engine is oil.

The 72-79 bus thermostat doesn't open until 85-90C (185-194F). When the engine is "cold" and it's 85F outside it is still 100F below thermostat opening temp. This is the simple reason why it is required for ALL CLIMATES: the warm up system has yet to activate even when it's too hot to go outside.

Even worse than a missing thermostat thermostat is the absence of cooling flaps. This is disastrous because on a Type 4 engine the flaps work asymmetrically (see photo).

In the photo, you can see that when the engine is cold, the passenger side flap is blocking airflow to warm up cylinder bank 1/2 but the driver's side is spitting the airflow. Air is bypassing the oil cooler to help warm up the oil quickly. When the engine heats up, the driver's side air flap lies on top of the oil cooler blocking access from above but it's still open to the fan shroud outlet just slightly below to direct air through the oil cooler.

Remove the flaps on a Type 4 engine and you've just removed the airflow from the oil cooler. The oil temperature will rise quickly but this won't counter the fact that the cylinders are being overcooled during warmup and the oil is not being cooled properly afterwards.

So there is a complex relationship between the oil temperature, the thermostat and the cylinder head temperature. Don't try to outsmart the VW engineers who worked on that engine design for 40 years. Granted they built an engine to be driven in all climates but if they could have saved $5 by leaving the part off, they would have done so and saved millions of dollars.

How does it work?

T4 thermostat

An air-cooled thermostat works nothing like the version in a water-cooled engine. Instead of being submerged in the engine coolant and subject to the coolant pressure and temperature, the air-cooled thermostat is mounted to the crankcase and just below the 1/2 cylinders. It is heated and cooled by proximity with the engine case and by the flow of cooling air that passes over the cylinders.

The thermostat is sealed brass unit with an accordion shape. When it is assembled at the factory, it is compressed in a jig, the air inside is evacuated and it is filled with a drop of liquid that has a boiling point that matches the number stamped on the body. The thermostat is brazed closed and permanently sealed.

When the thermostat is heated, the liquid changes into a gas and causes the thermostat to expand. Because the thermostat is in compression when it is sealed, there is less work for the gas to do in order to make it expand. When it cools, the gas liquifies and the air pressure inside drops and the thermostat compresses. There is no spring unless you consider the accordion is in tension when the thermostat is closed.

How does it fail?

A common failure that happens is when the cable breaks which is easily checked and replaced. It usually breaks where it's clamped at the shaft.

cracked thermostat crack closeup

If the thermostat is punctured, the liquid will leak out and it will expand immediately because when it is sealed the inside is at a lower pressure than the outside.

If you are not sure what size is normal you can judge them by these photos (the longer thermostat is the one that has failed). You'll also be able to flex the failed thermostat while the working one will remain rigid.

Thermostats can fail in one of three ways:

(1) Typically a hairline crack forms that is not easy to see unless you bend the thermostat in your hands and look carefully. The crack forms from expanding and contracting thousands of times.

(2) I've never seen one fail in the closed position and was doubtful when I first heard of it. I kept reading about this situation but the more I looked into it the more it appeared to be an urban legend based on the same source of the rumor that had been repeated over and over again.

Based on the way it is constructed I can only assume the the liquid inside reacted and became inert or it was that way from the time it was assembled at the factory.

Another theory is that some excess solder dripped through the cable nut end and has caused it to bind with the rod inside. This seems like a manufacturing defect because any solder that was melted by the engine should have caused the thermostat to leak and expand.

Those are the only explanations that seem reasonable because we know the thermostat has a tendency to expand when breeched. A couple of people have emailed me to let me know they've had this type of failure but I've yet to examine the thermostat (I still believe them). I suspect this type of failure is repairable but haven't verified it yet (see next failure).

(3) What about the third kind of failure? If the thermostat has expanded but you cannot find any cracks then you are in luck because you can repair this kind of fault yourself.


When the thermostat expands but not from cracking it is because of a failure at one of the brazing joints. Test the thermostat by submerging it under water and compressing it while you watch for bubbles to locate the point of failure.

cut open
Opening up the thermostat

Using a propane torch and a pair of needle nose pliers, heat the end cap until you can remove it (as shown in the picture). It's sometimes easy to spot where the seal has failed: bright solder everywhere around the circumference except in one spot where the copper has corroded.

Remove as much solder possible from both pieces by further heating, then quickly wipe it away with a cotton rag. The threaded piece sticking through the end (where the cable attaches) is actually hollow so heat it up and blow some air through to remove the plug of solder blocking it. Use a Dremel tool with a wire brush to clean up all the solder surfaces. Also clean out any dirt, water, solder, etc. which may be inside.


Apply some paste flux to the solder area, then clamp the thermostat in a slightly compressed form in a vise, then re-braze the seal using your torch. A mini butane torch is the easiest to handle. Just like a simple plumbing repair, if it's clean and there's enough flux, the solder will wick into the joint.

Refilling 80-85C thermostats

To make the thermostat expand at the set temperature VW used 99% isopropyl alcohol for 80-85C thermostats, which has a boiling point of 82C. This is commonly found at just about any pharmacy. Using a plastic syringe, squirt about 2 ml into the thermostat through the hole in the threaded piece.

Refilling 65-70C thermostats

This information is courtesy of Bryce Keba: "VW used wood alcohol (methanol, methyl alcohol), which has a boiling point of exactly 65C. I can confirm these findings because the smell of the freshly opened thermostat matched exactly that of a sample of wood alcohol vapor I distilled from cedar chips. The crude stuff I got out of trying to make it myself boiled at 78C, which negated the fact it was wood alcohol at all for the purpose of putting in a thermostat, so I searched for a cheap source of methanol in the US. After searching the far corners of the internet, I found that normal Heet gas line antifreeze (yellow bottle, NOT Iso-Heet) is 99% Methanol, and the best part is, it only costs $2 a bottle. I put 1-2 ml into the thermostat, and tested it in boiling water on the stove to confirm that it did indeed fully expand by exactly 65C."


Apply some paste flux to the solder area, then clamp the thermostat in a slightly compressed form in a vise, then re-braze the seal using your torch. A mini butane torch is the easiest to handle. Just like a simple plumbing repair, if it's clean and there's enough flux, the solder will wick into the joint.

Clamp the thermostat into the fully compressed position from the sides (measure it to be sure), leaving access to the threaded end which should be pointing up. A bench vice won't work for obvious reasons.

Heat the thermostat very gently so as not to melt the solder you used previously to seal the end cap and wait until some of the alcohol boils off through the hole. This has the effect of displacing any air which is left inside. Even though some of the alcohol has burned off from the heating, a little bit of alcohol should be left inside to expand it later.

While the thermostat is still hot, heat the threaded piece and reseal it with solder. Voila! Now go test it on the stove to see if your repair was successful (see measurements below).

Finding a NOS unit:

raiders logo

Unfortunately, the Type IV thermostat is no longer available (NLA means that OEM Wahler has stopped making them which I confirmed via direct email on 5/30/05).

The few that are left are often $50-120 which is a pity because Bus Depot used to sell them for $18-25 only a few years ago (back when more people thought they weren't necessary). If the demand had been as high as the ignorance back then, Wahler may have been able to continue production for a while longer.

Get one before they are truly all gone. If you can't find a bus thermostat you can convert a Type 1 thermostat for use on a Type IV engine. It's easy to do but keep in mind there are two Type 1 thermostats out there and they have different opening temperatures and the higher one is more ideal:

If you are looking on eBay and only find OEM part numbers Wahler 1104.80 is the same as 043-119-159 and Wahler 1106.85 is the same as 021-119-159A.

oem boxes markings

The reason 4 part numbers exist with only two temperatures is because the Type 1 style has a rod screwed onto it, which the Type 4 does not. Otherwise (except for activation temperature) they are identically constructed.

One other slight difference is that the thermostats that open at 65-70C have 13 folds, while the units that open at higher temperatures have 10 folds.

The best choice is to use the beetle FI unit because it's so close to the bus unit. I'm not sure why VW made the change part way through 1972, but based on the length all thermostats expand to when active (55mm hot vs. 35mm cold) it's ok to swap them.

See the link at the end of the article describing the conversion of a Type 1 thermostat for a Type 4 engine.

Stovetop Testing:

So how do you test your thermostat to see if it's working? First you should test for the existence of flaps if you aren't positive the PO didn't remove them. The passenger side flap is connected to the shaft, so if the shaft is missing then you have at most one flap and ZERO working flaps.

With the shaft installed you disconnect the thermostat cable and rock the shaft. You will hear the flaps hitting the tin indicating it's at the end of its travel. Because the driver's side flap is connected to the shaft by a linkage you'll need to find a way to use the spark plug hole in the tin to see if the flap is in place. It's awkward but that's the only option other than pulling the fan shroud temporarily forward.

If the thermostat under test is on the engine you can to start the engine and watch it expand. It can be a little difficult to watch unless you remove some of the tin underneath.

A simpler way is to simply check that the cable is attached and properly adjusted then watch the air cooling flap shaft to see if it rotates as the engine warms. When the flaps are closed the 1/2 (passenger) side flap is facing down and blocking the cylinders and when open it is horizontal.

If the flaps are already in the fully open position when the engine is cold you should remove the thermostat to see if it's already expanded and failed. You can see a NOS unit on the left and a failed unit on the right in the following photo:

A new thermostat is about 35mm long as measured in Bentley's diagram on page 23 of Engine & Clutch. When it hits its target temperature it measures about 46mm long and continues to expand until it's 55mm long. That's how much the cable that controls the flaps moves.

If you've bought a NOS thermostat and want to determine if it's good, besides checking the length you should give it a shake. If you don't hear a liquid inside it's probably bad.

If you are not sure you can test the thermostat on the stove. Fill a stove pot with cold water and lay a vegetable steamer inside. Place the thermostat in a ziplock bag and check that the water level is sufficient to submerge the thermostat. Monitor the temperature with a thermometer and heat the water until it boils.

At the temperature stamped on the thermostat the body will begin to expand. Resist removing the thermostat from the ziplock bag. If you do the thermostat will close up quickly and this is more stress on the accordion pleats than it was designed for. Let the water cool slowly (you can help it along by slowly adding cold water to hot).

stove testing open vs closed expanded during test

Misc Parts:

Except for the thermostat itself the other parts can be found new/used. The hardest item to locate is the union nut so try the Porsche dealer for that one.

The thermostat cable is often expensive for what it is. Luckily, it can be home made using some 1/32" stainless steel aircraft cable from the local hardware store and a crimped on ferrule.

bracket hardware

Cable adjustment:

Mounting the thermostat is simple but adjusting the cable takes a little more work. The first step is to mount your new thermostat as a complete unit.

adjusting spring

The rod that connects the left and right flaps has a fail-safe spring that serves to open the flaps against the tension of the thermostat. If the thermostat fails, the flaps will be held in the full-open position so the cylinder heads doesn't overheat. If the spring is not in place, the heads may overheat whether or not your thermostat is working.

I've adjusted enough thermostats that I can do it by myself but I recommend using a helper your first few times. A good trick involves clamping the rod with vicegrips to relieve hand strain while you work.

From the thermostat, thread the cable under the wheel and through the engine tin (there is supposed to be a rubber grommet in this location to prevent the cable from rubbing against the tin) and feed the cable between the square nut and the arm on the rod. Tighten the cable, make sure it has not come off the wheel and go for a drive to heat up the engine. If you don't have a CHT gauge to determine when the engine is hot, let the engine idle for 10 mins then go on a short highway drive.

Shut off the engine and check the tension in the cable. If there is too much slack there is a danger the cable may come off the wheel and then you will lose all tension. The engine will stay hot for a long time so you can undo the tiny 7mm nut and pull the cable through some more. Resist temptation to trim the cable (crimp a 1/32" ferrule on it instead if you want to clean up the look and prevent fraying of the cable).

Be sure to leave a little slack in the cable. Some people like to adjust the cable only when the engine is hot. That is, they remove all the slack from the cable in order to ensure the flaps opens fully. In my experience, there is no discernable difference between 95% and 100% open so by removing all the slack from the cable you ensure the thermostat will be under more tension when the engine is cold. In my opinion this is an invitation to crack the thermostat prematurely. You want just enough tension to keep the cable from coming off the wheel and to open the flaps.



06/12/05 - Created
12/8/06 - Added repair tips
09/07/11 - Fixed broken photos, added translate button, updated footer
06/09/13 - Updated to add Awesome Powercoat unit (thanks Lyle)
07/15/19 - Google update: new adsense code, removed defunt translate button
01/03/22 - Added Bryce Keba's research (thank you!)