How does one make sure that the thermostat works correctly? I know you can pour hot water onto it, but by how much does it need to expand and at what temperature? I have one and don't want to go through the trouble of fitting it to only find out it doesn't work and possibly fry an engine.

we talking new bug i assume?

the thermostats on the aircooled bugs in the fan shroud are very rare and were taken out on Aust bugs on the first scheduled service (or something like that as the story goes) as they were deamed unnecessary due to aust climate conditions, as opposed to UK and some US climates.

Understood. It would be getting a bit nippy out ur way nower days... my brother went out past ur way to Blood Wood creek for work last week and he said it was freezing.

I will check my old manual and see if it says anything. what yr bug is it?

http://www.vw-resource.com/vanes_thermostat.html 

You can test them with a hair drier BUT make sure its in its mounting bracket

if its not they can over expand and they're ruined.

a good one will be tightly compressed like this




and when hot expand like so



if at ambient temp it looks like this (expanded) then its dead

They're fail safe and expand when they puncture so it holds the flaps open amd doesnt cook the engine