Which technique is best for centering the tube shell gasket surface when preparing to spot a head?

The technique of using pencil wedges against flush bolts is the most effective method for centering the tube shell gasket surface when preparing to spot a head. This method allows for precise adjustment and stabilization of the position of the tube shell, ensuring it is correctly aligned and securely held in place. The use of pencil wedges provides a clear visual and physical aid that helps to compensate for any minor misalignments, ensuring that the gasket surface is properly seated and ready for the spotting process.

This approach enhances accuracy and minimizes the risk of errors that could arise from improper alignment, which is crucial for achieving a proper seal and maintaining the integrity of the system. The wedges can also be easily adjusted if slight modifications are needed during the setup, allowing for a more controlled and careful alignment process.

In contrast, other methods may not provide the same level of precision. For example, using a rubber mallet could potentially introduce unwanted forces or vibrations that might misalign the surfaces instead of centering them. Aligning by eye is often less reliable because it relies on subjective judgment and can lead to inaccuracies. Relying solely on caliper measurements, while useful for certain applications, does not account for adjustments in real-time that the wedge method allows for, thus making it less practical for