The thermostat is held in the housing by a steel band that is about one inch wide. The band is probably rusted so bad that it looks like it is part of the housing. Use a screw driver and a hammer to chip away the rust and you will find the steel band witch is probably rusted beyond use at this point in time. I have been buying a copper pipe coupling at the local hardware store, cutting it to the proper width with a hacksaw and bending it to fit inside the housing to retain the new thermostat.
