After a pipe and fitting are fitted, how should one measure the clearances between the pipe outside and the fitting inside?

Using a feeler gauge to measure the clearances between the outside of a pipe and the inside of a fitting is advantageous because feeler gauges are specifically designed to determine gap widths or clearances. They consist of a series of metal blades of varying thicknesses that can be inserted into the space to accurately gauge the size of the gap. This method allows for precise measurement, which is critical in brazing applications where the fit between components can significantly affect the quality and strength of the joint.

In contrast, rulers are primarily used for linear measurements and may not provide the level of precision needed for such small clearances. Calipers, while more precise, might not fit properly in tight spaces, and are better suited for measuring larger dimensions or outside and inside diameters rather than gap clearances specifically. Micrometers offer accuracy but are typically used for measuring the thickness of materials rather than the clearance between two surfaces. Therefore, a feeler gauge is the most suitable tool for this application in silver brazing, as it provides the right combination of precision and practicality for checking the necessary gaps between a pipe and fitting.