A thermostat is disclosed and claimed in which a totally enclosed switch is operated by a bimetal snap disc. The housing is provided with a small opening through which a testing probe may be inserted to test the gaging of the assembled device. Such opening is closed by a thin adhesively applied closure element, the application of which cannot alter the operation of the thermostat. A method of testing is also disclosed in which the thermostat is essentially fully assembled and then thermally cycled while a probe extends through a small opening to monitor the position of the movable element of the switch to determine the position within the snap range of the disc at which the switch closes. After the testing, the opening through which the testing probe extends is closed to complete the assembly of the device.