the wedge on the other side is probably stuck, unscrew the hex, then screw it just a couple of turns in and give it a light tap with a hammer, this will usually free the wedge and the adapter. if that doesn't work, take the hex out before you f*ck up the threads and tap the wedge out using something long (screwdriver, tap, piece of iron rod...)..
if the stem from giant was too big, it probably means that it was an 1" adapter with a shim for 1 1/8" stem, this is usually how they come...
the wedge on the other side is probably stuck, unscrew the hex, then screw it just a couple of turns in and give it a light tap with a hammer, this will usually free the wedge and the adapter. if that doesn't work, take the hex out before you f*ck up the threads and tap the wedge out using something long (screwdriver, tap, piece of iron rod...)..
if the stem from giant was too big, it probably means that it was an 1" adapter with a shim for 1 1/8" stem, this is usually how they come...