This is one I posted a while back on the vintageminiracing forum.
My oil pressure relief valve stuck shut and it wouldn't come loose. This happened at the track (of course) and I needed to fix it fast. I tried a magnet, didn't work. I tried shaving down a piece of wood and jamming it into the bullet, didn't work. So I made this expander thing from a bolt, nuts, a little piece of tubing, some washers, and a piece of fuel hose.
This thing worked. I shpritzed some brake cleaner in the bullet, then dried it out with a rolled up paper towel, and then used the expander. The two nuts on the end are tightened against each other, so you can hold it with a socket or vise grips. The next nut tightens down on the tube and washer, so the piece of hose gets squashed fatter, it expands and locks inside the bullet. Once it is tight, pull the whole thing out and it brings the bullet.