They are pigs to unscrew especially if it has been in a long time. My method is crude but works but you lose the thermostat. You knock in the centre past and use a small hacksaw blade to cut out a 1/8th of the remaining brass ring down to the thread with care and remove that segment then you can collapse the remaining body and it will undo cleaning the threads as it goes.
It is a good idea to have a high concentrate blue antifreeze to prevent further corrosion occuring in the engine if left empty but wet....
I agree with Tim, I had to use this method, you will probably need to clean the threads, but what hasn't been said is that the thermostat screws into the housing, which is why it's such of a pig to get out.
When you replace the thermostat make sure it has a bleed hole about 1/16" diameter, I had a couple of years of trying to figure out why my 2c kept overheating after replacing the thermostat, but there wasn't a problem when the thermostat was removed, in fact I ran it for two years without a thermostat and using a rad blind in the winter.
There was a bad batch of reproduction thermostats produced in the early 2000's which have been sold as old/new stock, I bought two thinking one was faulty and at about £50 a time it was expensive lesson, eventually figuring out what the problem might be, drilling the hole cured the problem and she's never overheated since.
When you drill the hole you need to screw the thermostat in to determine where to drill the hole.
Brian
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