Trakhak

Take it to an experienced mechanic in a good bike shop. The mechanic will have seen plenty of stuck-stem problems before and should be able to get the stem out without damage.

Carbonfiberboy 

Hey Danno . . . I don't have to make a drawing. I didn't think the attached photo was the same as my stem, but upon looking closer, I can see that it is the exact thing. If you look closely at the wedge, you can see the end of the horizontal cylinder I've been yakking about. The wedge, instead of being solid as most are, is hollow, with about a 3mm wall thickness. It must have a thinner wall thickness than the stem, because I can see the stem above it. I thought the stem end was another structure inside the wedge, but that must be incorrect.

The purpose of the cylinder is to give a true lead for the stem bolt, even as the wedge moves from back to front to tighten the stem into place. Nice engineering, really.

So yes, I just have to get over it and really whack on that stem bolt. With luck, I won't blow the cylinder out the bottom of the wedge.

I will try that right now, now that I know what I'm looking at.

DannoXYZ 

Ah, good job! That design appears to be for weight-savings and durability as well as the wedge is probably aluminium and the cylinder-insert is steel. As mentioned, most likely the aluminium wedge has corroded onto the steerer.

Carbonfiberboy 

Got it out!

Here's what worked and what didn't work:
Worked: Grabbed the top tube in the workstand clamp close to the head tube. Loosened stem bolt and whacked on it for all I was worth with a heavy hammer. After a few minutes it started to move. Used a 3/8" bronze rod for a punch so I didn't bung up the bolt head. Backed the bolt out a bit and repeated until I was down to a few threads.

Didn't work: removed the stem bolt, turned the bike over and pounded on the bottom of the stem close to the headset, using a long 1/2" bronze rod as a punch. No movement. Pounded on the cylinder part of the wedge with a 5/16" rod and heavy hammer. No movement.

Worked: Demounted bars, and heated stem close to headset with a propane torch, wide flame, for quite a while, until the metal near the carbon fork crown started to get warm. Heard a few cracking noises while doing this, good. Of course this ruined the headset, but I had no problem with that. Then poured cold water down the inside of the stem to cool it. Lots more crackling. Pounded on the wedge again from the bottom, with the 5/16" rod and heavy hammer. This time it moved about 1/16" after several minutes of pounding. After it moved, I put the bars back on the stem and remounted the wheel and torqued on the bars. The stem moved! Then there was nothing to it - came right out.

The corrosion was just at the top of the stem and the wedge. Most of the length of the stem never got Kroil on it, and still had some anti-seize on it, very dry and dirty. I think the main effect of the heating was to make the stem longer, and longer faster than the steer tube, partly because aluminum conducts heat so well and was not well bonded to the steer tube. That broke it loose so that pounding could move it. Once it moved, it was easy.

ultraman6970

congrats

Mr Danw

With the front wheel in place, cut the stem just above the headset.
Use a large punch and hammer to drive the expander cone out of the bottom of the steerer tube.
Remove front wheel and feed a hacksaw blade through the stem then attach to a hacksaw frame.
Cut through the side of the stem from the inside.
Rotate the fork 180 degrees and cut the opposite side.

Carbonfiberboy 

And thanks to all for their help and ideas!