Quote Originally Posted by cwilliamrose View Post
The other side of this is that gaps leave an exposed edge which the welder may try to protect by using less than sufficient heat. The weld gets wider because there's a gap to bridge but the penetration is not improved, it only looks that way because of the filler coming through the gap on the back side of the mating tube. The penetration on the 'main' tube (for example the longeron) is not enhanced in any way due to the gap being filled. The only way to get good penetration on both parts is to use the proper amount of current and good technique. Machine setup, tungsten grind, etc all play a role as well. Simply providing a gap does not insure a good result but can and usually does create more distortion in the finished weldment. There are no shortcuts to good welds and tight fits do not cause poor welds.

I agree that the bushing used in this mount is too thick walled and that extra metal serves no purpose. Getting enough heat into that bushing while dealing with a poor fit would be quite difficult.
Part of the purpose of the gap on butt type welds is to provide some gas coverage into the joint itself. It also allows the arc to reach between the two parts, something it cannot do when fitted up tightly. If gas welding the gap allows the heat to travel between the two parts. AWS D17.1 specifies the gap to use, FAA as well. Industry uses gapping. While it itself is not the root cause of this particular issue, it is by far the best practice.