The No. 245 and No. 330 burs differ from one another in which respect?

Correct answer: C. Head length — approximately 3.0 mm versus 1.5 mm

• A. The head diameter is the same for both at 0.8 mm — this is what makes the depth relationship so convenient.
• B. The head-to-shank relationship is identical.
• C. CORRECT. Both are PEAR-SHAPED burs with the same head DIAMETER of 0.8 mm and the same slight occlusal convergence. The difference is LENGTH: the 245 has a head about 3.0 mm long, the 330 about 1.5 mm. The 330 is effectively a shortened 245, which makes it well suited to the shallower preparations of primary teeth and to conservative occlusal preparations.
• D. Both are pear-shaped; neither is an inverted cone.

Note: The 245's 3 mm length is a built-in depth gauge: burying HALF the head gives the 1.5 mm ideal pulpal floor depth for an occlusal amalgam preparation.