I agree with the tool suggested by Mo-Betta. Sometimes I think what happens is applying extreme end pressure to the threaded portion, especially with a hammer, can cause the tapered portion to bulge and make removal a next-to-impossible proposition. This is my suspicion on a ball joint that I fought for two days, not that long ago. I had to resort to drilling the threaded shaft to make the "walls" weak enough to break it free.

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Sam

84 300SD 350K+ miles ( Blue Belle )