If a cast crown fits on the die but not on the tooth with a discrepancy of about 0.3mm, what should be done?

When a cast crown fits well on the die but has a significant discrepancy of 0.3mm when placed on the tooth, it indicates that the crown does not adapt correctly to the prepared tooth's contours. This can occur due to various reasons, such as an incorrect impression, errors in the die fabrication, or changes in the tooth being crowned after the impression was taken.

Taking a new impression and fabricating a new crown provides the most reliable solution in this scenario. It ensures that the crown will have the correct dimensions and fit, aligning properly with the tooth to ensure adequate function and aesthetics. This approach prevents potential issues like improper occlusion, marginal discrepancies that could lead to decay or periodontal problems, and overall patient dissatisfaction with the restoration.

The other options, while they may address specific issues, do not resolve the fundamental problem of fit. For example, burnishing margins is not effective for a significant gap like 0.3mm, and relieving the cast from the inside could compromise the integrity of the crown. Similarly, using a thick mix of cement is only a temporary fix that would not resolve the underlying issue of incorrect fit. This is why the best choice is to take a new impression and create a new crown.