Generally a radiater shop repairs with solder I believe, but believe it or not we just fixed a heater core with J.B.Weld. Block off one end of the core submerge it water with a little air pressure, find the leaking area try to open it up a little bit sit it on a warm wood stove and inject with a plastic syrenge some good ol STRUCTURAL j.b. weld. I'll let it set fo about a day or so than submerge it back in the water bucket for about an hour at 7.5 lbs. looking for leaks if she does I go back over it again until she holds, we've done a few of them like this.Some people say this could be dagerouse because of the weld softning up and giving off fumes B.S. J.B.weld doesn't even soften up on exhaut manifolds.For $7.50 she's worth a try
