Another stem won't come out thread
 

It's a Modolo Race aluminum quill stem in an aluminum frame with an aluminum steerer carbon fork. I need to replace the headset. I built the bike a few years ago and am sure I used anti-seize on it, but it's my rain bike. I knocked whatever the grabber thing is down fine - I don't remember if it was a wedge or a tapered plug. The stem doesn't move, though. I'm good with destroying the stem, but don't want to damage the fork or do something that will make the stem even harder to get out.

PB blaster or similar used copiously and prepare to wait for it to work. The usual recommendation of ammonia or oven cleaner won't work here because it would attack the aluminum steerer too.

Have Kroil, will begin chemical attack now. After a couple of weeks of Kroil, next step in attack?

I suspect that the corrosion in your case might not be as bad as with dissimilar metals as with an aluminum stem in a steel steerer tube. I've encountered some steerer tubes in forks that taper to a smaller ID towards the bottom, specially if the fork crown and blades are a different material from the steerer, as it is needed for ovelapping and bonding the two different materials together. It may be that your stem is just jammed in this tapered area and mostly just needs a little physical "persuasion" to remove. Is there a way to tap on it from the bottm upwards with a metal rod without having the wedge inside get in the way?

Chombi

The fork has the normal little hole in what would be the bridge. If I remove the brake, I can get something in there, but the wedge or plug will still be in there.

In looking at this option, I've come to the conclusion that the wedge didn't drop after all or maybe doesn't drop now. If I tighten the stem bolt and pour Kroil into this hole, with the bike upside down, it does not come out the top. If I loosen the bolt, it does. Bike is presently upside down with Kroil in the steer tube.

If the wedge bolt is long enough, you can usually loosen it to have the wedge hang down enough to develop space where you can sneak in a thin metal Rod to use to tap the stem out from below at the seam where the stem tube end meets the steerer tube. Just make sure that the rod is seated well on to the step where the tube meets before hitting it with a mallet or hammer. Be gentle and use good judgement on the intensity of your tapping on the rod to make sure you don't end up really bashing up the stem's edge and steerer tube's interior.

Chombi

I've attached a photo of the stem in question. Wedge takes up most of the bottom of the stem. Kroil has not yet begun dripping from the stem/headset joint, but I'll be patient.

Get a vise around the top of the stem and twist the sucker out.