I had exactly this problem a few years back! both parts alloy with a shallow thread and both easily damaged and expensive!

ACF-50 worked leaving it for a few days to penetrate, it was designed for treating corrossion in alloy wings of aircraft on aircraft carriers.

I've been using it ever since to protect and free up all sorts stuck bits of metal.

You can find it in many motorBike shops or look here.
ACF-50