Screw cap stuck in adjustable stem connection, how to remove or reattach?

Question

I was trying to detach my handlebar, because lowering was insufficient to make my bicycle fit in my box. I detached the long screw going connecting two lids thereby fastening the stem-handlebar connection, but the lid of the left side is stuck:

Can I use some gentle violence here, or do I need to unstuck the lid in a different way? I'm not sure if there is a thread inside the hole, and I'm not quite sure why the cap has ripples. I don't want to destroy anything.

Note: I'm not sure if my terminology is correct overall

Answer 1

This is your stem.

As you can see, it is the whole unit from the bars to the top of the fork, including the adjustable bit in the middle. Most dan't have this and I think this is where the confusion is coming from.

To remove your bars for packing, it is usual to remove the 4 bolts on the face plate on the stem. That is the part immediately touching the bars.

After you've removed the bars, you'll want to tape or strap them to the frame somehow so they're not dangling and pulling on the cables. You shouldn't need to remove anything else.

Do not remove the bars by undoing the 1 bolt that adjusts the angle, as you have already, I can't find any diagrams of how it all works.

Do not remove the bars by undoing the 2 bolts that hold the stem onto the fork as this will remove the forks as well.

Answer 2

I solved by detaching the other side and spraying the inside with WD-40. Then, simply tightening the screw on the other side led to the previously stuck side being drawn inward.

To detach my handlebar, I instead followed the suggestion in arnes comment.