Amir,
The technique you describe would have made tremendous sense
but Schwinn chose another method for making the head-tubes and
BB shells for their heavier frames:
http://www.sheldonbrown.com/
Fred Rednor - Arlington, Virginia (USA)
>
>
> Another way of making joints in tubes is to place a tube in a
> "T" shaped
> die, plug the ends, and pump fluid into the pipe under high
> pressure until a
> bubble forms in the T-opening.
>
> This is a standard manufacturing method in plumbing tube
> industry.
> The results are much stronger than a welded joint, but
> tooling costs are
> high.
>
> Schwinn may have used this method in their heyday.
>
> Amir Avitzur
> Ramat-Gan, Israel