Maybe you should wear tighter pants! *badum-tish*
Seriously though, head to your local auto parts store and pick up a $3 tube of epoxy putty (it may be labeled as 'Gas Tank Repair Putty'). It smells like burning hair, but it cures in 5 minutes, fully cures in an hour, and dries rock hard. Stick some on the threads, screw the balltop onto the shaft, and wipe off the excess putty. It'll never come off again, though, so make sure that you really want it to stay.