Pyoor's excuse for not working

In my experience the chemical metal should be able to cope with the petrol and heat. Fenchurch has survived over 20,000 miles with the stuff sealing a hole in the inlet manifold right next to the exhaust manifold. I broke through with the hacksaw when I sawed the one-piece manifold into two pieces - a temporary bodge that is working too well to bother fixing properly. Any idea what caused it to crack? If it's flexing somehow then the chemical metal might not be strong enough to hold the crack together.