Which bur is used specifically to produce convergent facial and lingual walls in an amalgam preparation?

Correct answer: D. No. 169 tapered fissure bur

• A. A round bur is used for initial entry and for caries excavation; it produces rounded internal form, not converging walls.
• B. An inverted cone bur produces walls that DIVERGE occlusally and is used to create undercuts and flat pulpal floors.
• C. A straight fissure bur produces parallel walls with no convergence.
• D. CORRECT. The No. 169 is a long TAPERED FISSURE bur, and it is the taper that does the work. Held with its long axis parallel to the intended path of withdrawal, the bur's tapering sides automatically cut walls that CONVERGE occlusally — which is exactly the retention form an amalgam preparation requires, since amalgam has no adhesive bond and depends entirely on mechanical retention.

Note: The direction of taper is the whole point: amalgam preparations CONVERGE occlusally for retention; cast gold inlay preparations DIVERGE occlusally (2–5°) so the casting can be withdrawn and seated.