Good brazing depends on the ability of capillary action to draw the molten brazing filler metal (BFM) in all directions throughout the joint being brazed, either vertically or horizontally. In many brazed joints there is often the need for the BFM to flow upwards against gravity, and if the gap clearance is properly made, this should not be a problem. However, if the gap-clearance between the faying surfaces in a joint to be brazed is too wide, then gravity will dominate the situation, and capillary action may not occur. Therefore, it is very important for all brazing shops to know the “capillary-capability” of each of the furnaces they use for brazing. This is not hard to do.
Optimal gap clearance varies according to the combination of base metal and BFM used; e.g., copper BFMs prefer tighter joint clearances than for aluminum BFMs. Also, depending on the reactivity of the BFM with the base metal being joined, you may find that a given BFM will prefer either a tight gap or a wide gap. NOTE: By reactivity, I mean the ability of the BFM to alloy with, and diffuse into, the base metal. You will also see that this reactivity will affect the distance that a BFM is able to flow into a joint being brazed.