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Any suggestions?

with it clamped upside down with a rag in a vise, drawfile it to remove the chrome/level the gasket surface. Might drawfile the stat neck on the pump housing also (put a rag inside to keep filings out) but very likely the issue is the chrome itself on the stat housing & less likely is it being slightly warped, just not completely flat but drawfiling takes care of both. The sides and top (that you can see) will still be shiney chrome so all is good in the 'hood

live every 24 hour block of time like it's your last day on earth