In a message dated 12/31/2003 8:13:37 AM Central Standard Time, 13brv3@bellsouth.net writes:

<< Thanks Perry.  BTW, where are these RB welding instructions?  I just looked
through the catalog, and also the web page trying to find any mention of
this.  All I found were pictures of RB headers that were welded on the
outside.  Thanks,

Rusty
 >>

It is just good practice when welding up headers. More important on aluminum heads or Rotaries. The flange is clamped against cold (relatively speaking) aluminum and the exhaust gasses in the 1600 degree area heat the tubing to a dull red just past the flange. So if the if the tube is welded only on the inner surface, the tube can run at a uniform temperature in the first 6 or 7 inches with little stress. If you weld around the outer side of the flange. The tube is still at 1600 degrees and the flange still at say 240 degrees. Then there will be some serious stress develop at the weld junction. Thin walled tubing will tend to crack through the outer edge of the weld bead.  This is not a guarantee that the weld still won't crack. It is just less likely, and the result might cause less damage. Check pipes in preflight. Cold air will be sucked into system and you may detect a popping or a backfire on closed throttle.

It works well so long as none of the tube touches the sides of the hole through the flange. In the case of thick walled tubing, it doesn't matter.  Don't braze anything on a header system. Lynn E. Hanover