rocker arm vs valve rocker

rocker arm and valve rocker both are nouns.

Word	Noun	Adjective	Verb	Adverb
rocker arm	Yes	No	No	No
valve rocker	Yes	No	No	No

As nouns, rocker arm and valve rocker are synonyms defined as:

rocker arm and valve rocker: a lever pivoted at the center; used especially to push a valve down in an internal-combustion engine

rocker arm (noun)	valve rocker (noun)
a lever pivoted at the center; used especially to push a valve down in an internal-combustion engine	a lever pivoted at the center; used especially to push a valve down in an internal-combustion engine