My first 0551 had minor play - but the blade was almost against the G10 side. It was the only one of the six my enabler had like that. So, when I grabbed one to purchase, which one did I grab? Yep. No problem, < a 1/4 turn CW on the pivot screw (T8) centered the blade and 'removed' the blade play - KEEPER! That was months ago - early in it's availability (#057x). I bought my second one Saturday - perfect. I had to know - so I loosened (CCW) the pivot screw 1/4 turn - same result as my original 0551 - blade near the G10 liner and with some blade play. I returned it to it's original position - 1/4 turn CW - centered and no play. If you have 'play' when locked - and the blade isn't centered when closed, ie, is nearer the G10 side, tighten the pivot screw. Too tight, however, and it won't open! If that doesn't apply to your 0551, etc, I'd send it to ZT.
Stainz
PS My new 0551 was s/n 052x - lower - go figure! Did I mention what a keeper they are??