The images posted by Mariano and Jacques are of the original "Bellows" design thermostats. They fit snugly into the outlet of the head and don't pose much problems with shifting. But these are no longer manufactured in that style anymore, with only 2nd hand or NOS available, which may be 50 years or more in age.
The modern style of thermostat pictured below are all that are made these days, and can be specified in a wide range of suitable temperatures. However, without the cylindrical body of the Bellows type, they do not locate quite as well in the head outlet, and need some sort of physical restraint to stay put ON the outlet, otherwise they lift off and can allow coolant to flow past it.
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