First file the broken nub flat, leaving as much material as possible. Now get out the 3/8in. NPT tap and keep moderate pressure and tap away. Now you have this.....

Now cut down a 1/4"x1-3/4" pipe nipple right at one threaded end....

Then coat the nipple threads, and expansion tank threads with you favorite sealant, I used a product called PC-11, but I would guess that J-B Weld would work.

Now let it all dry and off you go. So far after going through a couple heat cycles with the car parked and it works like a freaking champ.
Sorry all the pictures stink but they were taken in the back of a box sitting on my bed.
If only they had a happy little school girl dancing smile you would know how I feel when I get to drive this...















































So there you go, when I get a chance to test it on the road I will report back.
Matt