The outline of the isthmus width should not be wide! If it extends beyond an imaginary line that divides the cusp tip and the central groove, it is very wide already. Doing this will surely undermine the cusp and lead to fracture.

To be exact about the measurement, it should be limited to 1mm in width. Use your bur #56 (diameter = 0.9mm) to glide and pass through the occlusal aspect of the preparation. It should pass smoothly with minimal to no resistance along the walls and isthmus width.

Using of the hand instruments is not advisable because every manufacturer’s measurements are not that very consistent, you’ll either get a wide instrument or a thin one. But burs have consistent diameters. So, in whatever situation you’re in, burs are always reliable as a measuring tool.

Using a #56/330 bur to measure (D= 0.9mm) – photo