Off o' tha top o' me 'ead the key is square - 5 tumblers, 5 levels. If memory serves, you will need 5 tumblers (fewer, if some can be reused), one tumbler retainer, plus any tumbler springs that you lose, and possibly a cover. It's not at all difficult, if you can find the small parts. That said, you maybe better off taking the lock to a local smith for rekey.