Pinch bolt on the exhaust flange

[Donald A]

I have 3 featherweight drive shaft casings, which I am trying to disengage from the exhaust flange - so as to get the casings rechromed.
2 still have the gear box attached - the 3rd only has the flange, minus bolt.
I have tried a C200 camping gaz blowlamp, on the premise that the aluminium alloy should expand more than the steel pinch bolt and steel casing tube, without success. What I don't want is to shear the hex head, because I don't have a bench pillar drill to drill out the remaining bolt shaft. I thought that with enough heat, the whole thing would be fall apart.....and this is meant to be one of the easy ones!!
Am I on the right lines? Should I just keep going and heat it for longer? Or should I buy a Bullfinch type gun torch to go on the propane cylinder I have left over from caravanning days, if this is hotter. I don't want to crack the aluminium
Donald A