OK............... color me stupid again! The M5 is correct. Tried a couple of M5 and couldn't get it to start on the thread. Finally looked at the stud real close and there was a nub/burr at the beginning. After carefully filling it down with an emory board it threaded right on and clamps fine. Sometimes you just can't see the forest for the trees. I need to re-think my thought process from time to time and look for the simplest most obvious cause. GREAT forum and GREAT members willing to help.
THANKS TO ALL!