Black and Decker GL565-HOW DO I STOP THE COMPLETE HEAD UNIT FLYING OFF THE?

Hi.The problem I have is that the complete spinning head unit flies off my ancient B&D GL565 grass strimmer.Looking at the exploded parts manual everything is there but the alloy "cap" would appear to be part of the motor extension shaft but is not a tight fit and does not appear to be permanently fixed in any way to the main shaft.All the head parts are screwed onto this "cap" so if it comes of the motor extension shaft everything is bound to come off the strimmer.Question is --how do I tighten/affix the"cap" back onto the extension shaft?