As a last ditch effort I have used a cutoff wheel and made a cut thru the aluminum down to the threads. At this point you can spread it a part enough to loosen it. Once reassembled on the pump you can't see it. If you want, you can repair your handy work with JB weld and nobody will be none the wiser.

Always buying gas pumps and parts.

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Buckles Vintage Gas Pump Parts LLC
Carthage Indiana