"Clays has an iron chin". That is wrong. Iron is much softer and much less resilient than Guidas chin. Add enormous hearth and will to fight, and its no wonder that he never was KO'd, and surely its doubtfull that anyone in lightweight can knock him out.
Awesome stamina, he does not even slow down during the fights.
..and a great guy.
A damn shame that he has a horrible "Stand-up" and average striking, cause he could threaten the very best fighters otherwise and even win the belt.