-SG- I recently asked David in New Zealand for his views on frame tube replacement - some advice on this subject has been published in the past in both books and magazines. Most of the suggestions involve drilling out the lug locating pins and then applying heat to soften the original braze to a level where the tube can be withdrawn. David's procedure involves removal of the old tube by appropriate machining.
He says: "About the frame tubes... I don’t drill out the pins and use heat. The casting sockets are often quite thin and prone to splitting when heat is used. I cut the tube 4-6” away from the end of the socket joint and use the tube to give alignment. I set them in the mill and bore out the old tubes in successive cuts. This way you can be sure of removing the vast majority of the old tube and obtain a clean surface to braze to.
Prior to brazing I use a flap wheel inside the socket to provide a relatively smooth surface. Cleanliness is essential. Wipe out the socket and the tube with acetone removing any residue, and then paint the joint surfaces with flux. I make up a thick slurry of brazing flux and water for this purpose.
I use a brazing filler rod which has a small percentage of nickel. It is an old product trade named “Steelbraze”. This small amount of nickel aids the capillary action, I can draw the filler up a 2” underhand socket joint with this filler rod. If Steelbraze or its equivalent is not available then a 15% silver-solder rod would work. Use a high temperature flux like Stay Silv. If using Steelbraze allow about 0.008” clearance in the joint for the filler, if silver-solder then 0.005” is sufficient.
I always jig frames to hold them true. The original method of using pins to hold the alignment worked with powdered flux and a brazing hearth as the joints had minimal clearance, but is insufficient for a looser hand brazed joint."