Start by sawing it lengthwise as far down as you can go.
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Bust off half with a chisel.

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This one's almost off. The other side is next.
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Take the blade out of the saw frame, and cut parallel trenches in the fitting. Pay close attention so you don't cut into the manifold threads - you want this to seal again!
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Make a few cuts around the inside of the fitting. Then start banging around the outside of the fitting with the chisel. When you find a weak spot, collapse it inward. Work your way around it, peeling it off the inside of the hole, and eventually it will fall out.

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Look at those nice intact threads! Clean with a wire brush, ,and chase it with an NPT tap if you have one.

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All ready for a new fitting. Use brass so the next poor bastard doesn't have to go through this too.