This study compared the preparation of long-oval root canals using the Reciproc single-file reciprocating system versus the BioRace multiple-file rotary system through micro-computed tomography. Distal root canals of mandibular molars were prepared with each system and scanned before and after to measure volume increases and remaining untreated canal surface area. The Reciproc system resulted in greater volume increases in the full canal and apical third, but left more untreated areas in the cervical and middle thirds compared to BioRace. Neither system was able to fully prepare the long-oval canal shape. The Reciproc system removed more tooth structure while BioRace left fewer untreated dentin walls in the more oval thirds of the canal.