WordNet records no synonym and no antonym for petcock. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
regulator consisting of a small cock or faucet or valve for letting out air or releasing compression or draining
Synonyms, antonyms and the broader and narrower relations for petcock come from Princeton University’s WordNet. Full sources and licences.