Tube leaks that cannot be cured by re-expanding can be dealt with permanently using Loctite 290.

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In the picture below, the tubes indicated by a red mark are leaking. The three with green marks were previously leaking and fixed with 290. They remain fixed a full year of hard use later.

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Loctite 290 is the thin wicking-grade of bolt locker. Stand the boiler on end and apply a little Loctite to the leak location (which should be dry) and allow to soak in and set. A second application a day later will do no harm. The temperature tolerance of the 290 is just high enough that a boiler in steam does not exceed it.

This technique (I am told) will also work to take care of pinhole leaks in a silver soldered copper boiler. Penetration can be assisted by applying a partial vacuum inside the boiler.

Click the thumbnails for a larger view.