My father has a radiator in his house that is missing a Thermostat Unit (the plastic thing that screws on top of the valve - not really sure what it's called). I managed to screw on a unit from another radiator but once I try to turn it (to switch off the supply) it just pops out of the silver nut that's supposed to keep it in place.
I have some pictures of both radiators (last image shows the problematic one) and it seems that the valve is slightly different and also doesn't seem to have that dark ring with grooves (which I guess is what holds the Thermostat in place).
I tried searching on the internet to see what this ring is but it's quite difficult without knowing its name. Anyone know what it's called or perhaps the valve is fine but I need a different type of Thermostat Unit to go on top?
Any help would be much appreciated. Thanks