There are three different thread sizes and pitches involved with the grooved nut/prechamber assembly.
First is the rather coarse internal thread of the nut, this what the injector screws into. Second is the coarse internal thread cut into the head. This is where the grooved nut screws in. Third is the fine thread cut internally on the prehamber itself. This is not visible with the grooved nut on place, but it is this fine thread that must be used to pull out the prechamber. I used a bolt to pull with rather than a slide-hammer because you can get much more force that way.