Crown preparations are the exercises most bench tests are built around, and the ones where marks are most often lost. Whether your programme sets a full gold, porcelain fused to metal or all ceramic preparation, the same small number of things decide the score.
Margin placement and finish
Your margin needs to be smooth, continuous and clearly defined. Most schools expect it to sit 0.5mm to 1mm above the gingival contour.
The mistake that costs most candidates is trying to fix a rough margin by polishing it repeatedly at the end. Each pass removes more tooth. The margin becomes jagged rather than smoother, and it creeps towards the gingiva. Damage the gingiva and you have lost marks twice from one habit.
Finish the margin properly as you go, rather than treating polishing as a rescue.
Reduction
Different crown types need different amounts of clearance, and your programme will have a guideline for the preparation it sets. Learn that guideline for the specific exercise rather than working to a general figure.
Then verify your depth more than one way. A putty index is the usual method, but graders can also judge reduction by comparing your preparation against the adjacent tooth. If those two readings disagree, that is noticed.
Taper
Taper problems come from handpiece angulation and the absence of a fixed reference point. Without those, the walls end up wherever the hand takes them.
Over-tapering loses retention form. Leaving walls too close to parallel causes its own problems, and it happens most often on bridge preparations where candidates chase a perfect looking result. Fix the angulation and the reference point and taper becomes repeatable.
Protecting the adjacent tooth
Interproximal reduction is where adjacent teeth get nicked. It costs marks, and it is recoverable.
What is not recoverable is quietly polishing the damage to hide it. That almost always causes more damage. Tell the faculty instead. Identifying your own mistake is treated very differently from concealing one.
The temporary crown
If a temporary is part of your exercise, it is assessed on fit, margin, contour and finish, and it is where timing shows. Most candidates have never made one under exam conditions. Practise it before the day, under time.
Practising crown preparations well
Repetition alone is not enough, because you cannot see your own preparation the way a grader does. Work through preparations in sequence, have the result assessed, then repeat with that feedback in hand.
For the wider list of what goes wrong, see why candidates fail the bench test and how the bench test is graded. The course with Dr. Kaur works through full gold, porcelain fused to metal, all ceramic and bridge preparations with evaluations on your own work.