We found one dictionary that defines the word countersink bits:

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1. countersink bits: Collins English Dictionary

(Note: See countersink_bit as well.)

Definitions from WordNet (countersink bit) ▸ noun:  a bit for enlarging the upper part of a hole ▸ Also see countersink_bit ▸ Words similar to countersink bits ▸ Usage examples for countersink bits ▸ Idioms related to countersink bits ▸ Wikipedia articles (New!) ▸ Words that often appear near countersink bits ▸ Rhymes of countersink bits ▸ Invented words related to countersink bits