>rock hard substance absorbed fluid and became  spongy! I personally
>would not use it to repair aluminum heads in contact with anti-freeze
>because of this. To be clear I am talking about the epoxy-like substance
>that comes in two tubes and must be mixed.

My '87 has had JBWeld'd heads for 4 years, I've owned it 2yrs/25K miles and 
still bone dry.  My '85 has 5 years and reportedly 75K miles since redone.  
The mechanic here has done 7 engines, both heads, and claims only one 
failure due to too much trapped epoxy thus pressure on the gasket splitting 
it during tightening. I've also remounted the alternator bracket bolt with 
JBWeld. It was drilled through, right into the waterjacket and leaked like 
hell when I bought the '87.  Two years ago also.  For $6.95 a tube count me 
in on my next set of heads!   tim smith