The thermostat on a Massey Ferguson 231 tractor is located at the bottom of the radiator. It is held in place with a small clip. To replace the thermostat, you will need to drain the radiator, remove the clip, and then remove the old thermostat. Once the old thermostat is removed, you can install the new thermostat, re-install the clip, and then refill the radiator. Make sure to follow the instructions provided by the manufacturer, and use new coolant if necessary.