fast engine warm-up.

The first thermostats used a sealed capsule of an organic liquid with a boiling point just below the desired opening temperature. These capsules were made in the form of a cylindrical bellows. As the liquid boiled inside the capsule, the capsule bellows expanded, opening a sheet brass plug valve within the thermostat. As these thermostats could fail in service, they were designed for easy replacement during servicing, usually by being mounted under the water outlet fitting at the top of the cylinder block. Conveniently