J-B Weld is a two-part epoxy-type adhesive known for its high tensile strength, especially when used on metal parts.
There are many examples where farmers repaired cracked engine blocks on tractors using J-B Weld. In fact, if you're curious about how good this repair compound is, go on YouTube and search for "jb weld" and "project farm". He has several videos on the product, but the one I would recommend would be the one where he uses it on a cylinder head of a lawn mower. Project Farm is well known as a small engine and mower one-man destructive force. To test J-B Weld, he cut holes in an aluminum cylinder head of an old lawn mower after he had calculated the cylinder pressure. After angle grinding the cylinder head, he repaired it using J-B Weld and let it sit long enough to cure. Then he proceeded to mow a lawn with the now-repaired mower.
With that said, J-B Weld is really supposed to be a temporary fix, at least on paper. If your repaired object has high pressure and temperatures, it may flake off in pieces due to the different thermal expansion properties of the repair and the object itself. Farmers, being practical, will wait until the J-B Weld finally starts to degrade on their engine block repairs and call out the local welder to make a more permanent fix. Sometimes that takes years to happen.
Since J-B Weld is so inexpensive and readily available at home repair stores and Walmart in the United States, it's a good thing to keep in your kitchen junk drawer for the time you may need it. I've used it on copper pipes after punching a hole when hanging heavy paintings. Worked like a charm and never leaked.
Noder npecom notes "When used in repairs that will be exposed to high temperatures it helps to get J-B Weld's Hi Temperature Resistant products"