Yeah.. the OP must be hell bent on saving the stem?!

One time i soaked it with a mixture of transmission fluid and acetone for a week, it still wouldn't budge so i belted the stem down a little bit and it moved! {bit like a frozen nut technique.. if you tighten it up a tad it will loosen** Then clamped it in a vice.. twist it left to right in small motions at first.. adding upward pressure.