PurposeVibration from the high‐speed handpiece may adversely affect the cement layer between a casting and a tooth, causing loss of retention or increased microleakage. This study assessed the effect of crown preparation on retention and microleakage of cemented cast post and cores.Materials and MethodsPost and cores were fabricated for extracted teeth and cemented. Specimens were prepared for crown restorations, varying the preparation time and the rotary diamond instrument used. After thermocycling and immersion in a dye bath, tensile unseating force and linear dye penetration were measured.ResultsIn general, retention decreased with increased severity of crown preparation; however, differences were not statistically significant (α= .05). Although the differing preparation conditions resulted in significant differences in microleakage, no consistent trends were evident.ConclusionsCrown preparation using high‐speed rotary diamond abrasives of cast post and cores luted with zinc phosphate cement had no significant effect on their retention, but had varying effects on subsequent microleakage at the cement‐tooth int