countersink vs countersink bit

countersink and countersink bit both are nouns.

Word	Noun	Adjective	Verb	Adverb
countersink	Yes	No	Yes	No
countersink bit	Yes	No	No	No

As nouns, countersink and countersink bit are synonyms defined as:

countersink and countersink bit: a bit for enlarging the upper part of a hole

Other synonyms of countersink include counterbore.

countersink (noun)	countersink bit (noun)
a bit for enlarging the upper part of a hole	a bit for enlarging the upper part of a hole
a hole (usually in wood) with the top part enlarged so that a screw or bolt will fit into it and lie below the surface

countersink (verb)	countersink bit (verb)
insert (a nail or screw below the surface, as into a countersink)