radiator cap

English Thesaurus

1. cap on the opening in the top of a radiator through which a coolant liquid can be added (noun.artifact)

hypernym	:	cap,
definition	:	a top (as for a bottle) (noun.artifact)

part meronym	:	radiator,
definition	:	a mechanism consisting of a metal honeycomb through which hot fluids circulate; heat is transferred from the fluid through the honeycomb to the airstream that is created either by the motion of the vehicle or by a fan (noun.artifact)