Drill the head off the bolt, place a rod (eg another bolt) on top of what is left of the original one and give it a good whack with a hammer. I can't see any reason why it wouldn't release the stem.
@moog started
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Drill the head off the bolt, place a rod (eg another bolt) on top of what is left of the original one and give it a good whack with a hammer. I can't see any reason why it wouldn't release the stem.