Originally Posted by T-Mar

Making the post Peugeot specific would not have precluded any issue, as the Peugeot PH501 was not TIG welded as you suggest. These frames were internally brazed using doughnuts of brazing material that were inserted into the ends of the mitred tubes. When heated using a ring torch, the brazing material material reflowed and some was sucked through to the outside of the tube by capillary action. The resulting joint had a large fillet on the inside with a smaller fillet on the outside. One of the major benefits of this method was that it allowed for visual inspection of joint quality. The presence of the fillet on the outside indicated the quality of the reflow via the size of the fillet and the amount and size of voids.