The most impressive fix I did with JB Weld was on a pair of cast Rover V8 exhaust manifolds. On the MGV8 the downpipe flanges initially were too thin, and they used to crack around the studs. Both my flanges had cracked and one flange had two pieces broken off. I put them back together with JB Weld, not expecting miracles, and yet these repairs held for several years. I eventually replaced the cast manifolds with tubulars and sold the manifolds on, making it clear how they had been repaired.
The only thing to watch with it is to allow plenty of time for it to harden off.
