Answer:
i have used it to fix a loose exhaust valve guide in the head of a car and to repair a thermostat housing. I have found that if you prepair the surface correctly it adheres well and sets very hard.
However, JB does not suggest using its product when the temperature will exceed its maximum temperature for more than 10 minutes. Will the surface exceed 600F? Don't use it. JB Quick has half the strength of regular JB Weld. If you can wait for the regular stuff to cure, you will be way better off. Also, the JB quick is half of all specifications, including temerture and stregth of JB weld, so if you do it at all, do not use JB Quick, use JB Weld.