As used in thermal switches. Instead of in form of a bimetal strip
and continuous curving with temperature, these are 0.5" diameter
and make a definite flip from concave to convex at a specific temperature.
About 1mm of abrupt movement gives a very positive transition for the
switch contacts. Would it be possible to change the characteristic
temperature by grinding back a spot/ring/radii? on one face? which
face? or would it just destroy the action/activation force?

Heaven knows, I don't like reinventing the wheel, but unless you can find
documentation (such as a patent) that explains in detail how these devices
work, and how a particular temperature is set, you're going to have to
experiment.

I again recommend looking for one of those interchangeable thermostats.
Perhaps ripping it apart will reveal the secret!

http://www.gopresto.com/products/pro...hp?stock=07211

http://www.acehardwareoutlet.com/(ua...?SKU=998000074

http://www.advertisingcookbooks.com/si/004123.html

The Presto "Control Master" has been around at least 40 years, and Presto is
still making appliances that use it. (I don't know whether the internal
design or operating principles have changed. The probe part does appear to
be longer and narrower in the current version.)