Even if the tube ends up having an eighth inch play or slightly more,
Corky writes:

it doesn't matter. You will be putting enough filler material on the
weld that the fuselage won't care, and no one will be able to tell
that it wasn't precisely flush. Won't matter in terms of strength and
safety either.


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I try to keep the gap to the diameter of the filler rod or less. In fact, I've
found a tiny piece of filler rod or snippet of MIG wire to be a handy means of
wedging a tube in place.

The typical fillet produced by gas welding is 3x to 5x the thickness of the
wall, depending on the included angle of the joint. Anything more serves no
purpose since that is all it takes to equal the strength of the tube. Some
amount of filler is required but standard practice is to keep the gaps fairly
small so as to conserve weight.

Overall, I've personally never found it to be a major concern. Some of my
welds are prettier than others but all are sufficiently strong. The only
definitive study I've seen comparing TIG, MIG & gas for welded tubular
structures was a thing for helicopter tail booms. All met spec for strength
but they went with MIG. It was fractionally heavier but it was faster,
produced the boom at least cost. Someone like NASA, with a virtually unlimited
budget but critical weight constraints, would probably have gone with TIG.
Home-builder or someone doing repairs, O/A will usually win the Practical
Factors test.

Folks who get all excited over things like welding procedures or 1020 vs 4130
are usually telling us more about themselves than about airplanes :-)

-R.S.Hoover