My passat repair manual states this:
The vent "valve" (seems not only a hole) helps bleeding the system, or
at least helped getting accumulated air out into the coolant reservoir.
Holes always need to be in the 12 o´clock position.

Sorry for the german text, but you´ll get the idea:
http://www.v6bus.de/thermo.JPG

Raimund

Am 23.04.2021 um 22:08 schrieb Jim. Felder:
> These thermostats with a hole were common 20 years ago but I have not seen
> them since. It seems the better German ones had the holes. I know of a shop
> that still drills Vanagon thermostats, their stance is that the hole helps
> pre-warm the cold coolant on the other side of the thermostat and prevents
> air bubbles in the system by equalizing pressure on both sides of the
> closed thermostat. I personally see no need to drill them or to go out of
> my way to find drilled ones.
>
> Jim
>
> On Fri, Apr 23, 2021 at 11:42 AM Dan N <dn92610@gmail.com> wrote:
>
>> thank you Dennis,
>>
>> I think the thermostat in there doesn't open fully. When the engine is
>> warm, usually the coolant temp. needle sits on the bottom side of the red
>> LED, now it sits a bit higher on the other top side of the red LED. When I
>> open the heater at full the temp. needle doesn't move down at all. It's
>> working, no overheat but I feel that something wrong/lazy with the
>> thermostat. I could be wrong but I don't feel comfortable with what I see.
>>
>> dan
>>
>>
>> On Fri, Apr 23, 2021 at 9:25 AM Dennis Haynes <d23haynes57@hotmail.com>
>> wrote:
>>
>>> What problem are you trying to fix? As the Vanagon cooling system has so
>>> much reserve capacity especially during cold weather getting the engine
>> up
>>> to temperature is actually a concern. The latest OEM thermostat,
>> revision F
>>> not only does not have a bypass, the top throttle plate is actually
>>> gasketed to prevent any bypass from the radiator until it actually moves
>> to
>>> open. With a good stat this begins around the 87C. If you really want a
>>> bypass open the bleed valve at the top of the t-stat housing.
>>>
>>> Dennis
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: Vanagon Mailing List <vanagon@gerry.vanagon.com> On Behalf Of Dan
>> N
>>> Sent: Friday, April 23, 2021 12:07 PM
>>> To: vanagon@GERRY.VANAGON.COM
>>> Subject: Thermostat with a hole?
>>>
>>> hi all,
>>>
>>> I am planning to replace the thermostat on my tintop syncro 2.1 WBX. I
>>> will get an 80° thermostat.
>>>
>>> When researching the topic on thermostats I found that some of them have
>> a
>>> tiny hole on it (about 1/16" diam.) and some of them don't have.
>>>
>>> Is there a pro and con idea of having one with the 1/16" hole - or even
>>> drill one hole?
>>>
>>> thanks for your help,
>>>
>>> dan
>>>