Tonight I went to pull the crankset off my bike so I could change the chainring bolts and this happened. That bottom flat piece of metal is supposed to be a taller cap, known as the crank puller cap (see crankset instructions here). It sits on the outside of the crank bolt, and as one undoes the crankset bolt it pushes against the cap, which pulls the drive-side crank off. Instead of doing that it just came apart, breaking off at the first ring of threads. This is not supposed to happen. Not much force was needed to break the part and the splines of the crank were well greased when I reassembled everything a few weeks ago, so I figure the cap simply failed.
While I can get another cap, there is a relatively thin cylindrical threaded piece of aluminum stuck in the crankset. I’m not completely sure how to get it out. I also can’t wholly disassemble the crankset until this piece of metal has been pulled and a new cap acquired. A very large easy out may work, but I might end up having to make something which digs into the remains of the cap and can unscrew it. It’s not very tight, but its not moving.
UPDATE: After a bunch of effort I was able to get the cap out and use a crank puller to remove the crank.