The cases would have been annealed between 300c and 410c, ( around 350c if you're using soap as a temperature telltale ) but likely the crystalline structures of the castings got a tad 'upset' by the combustion of the motorcycle. Petrol fires can generate temperatures of up to 2138c depending on the oxygen and fuel supply. I imagine the cases got only a light toasting though, otherwise they would have transformed into a puddle! Apologies for the anorak response, but my work involves the delivery of avgas which requires ADR category 3 qualifications (bulk flammable liquids).