The 'new' bolt is 5/16", not 1/4" (and not 8 mm).

There was a small roundish chunk of metal in the hole when I removed the bolt. I assume it was in there already but tough to imagine there was enough room for it and the bolt. I removed it with a magnet. Remnants of a broken bolt perhaps?

I'd like to get the injector (#2) out to further investigate but it's pretty stuck. I had #5 out and it was able to pull it out by hand. This one will be more difficult.