I'd advise leaving the cartridge assembled. Opening them up is fairly advanced, not strictly necessary, and it's not intended to be done as regular maintenance (hence the use of peening to hold the body cap on). Of course, if you were fooling around with valving, etc., then you would need to. If you remove the peening, use either Loc-Tite or re-peen in a new spot. I'd use Loc-Tite.