[woodiewagon46]

You are dealing with two dissimilar metals. The cap is cast and the tube is mild steel. The only proper weld, would be to TIG weld it with Ni rod. This will generate a lot of heat so if you went this way, I would do as John suggests and submerge the cap. It can also be brazed or silver soldered the same way. Tricky, but it can be done. Don't waist your time with a JB Weld type of repair. If you are going to do it, do it right.