what is it called?

i cannot give you a name for this device but i suspect it is viewing chamber for checking visually for air locks or preventing air locks to connecting parts. namely heater or cooling devices. if main expansion tank has seperate pipe to engine(thermostatt)and this vessel is unconnected to engine then try caravan centres (heating)rather than garages as it is a appliance adapter not a vehicle part.

james joseph
August 2006