Jeff Nelson presented a recap of the tolerance issues. Near waviness spec has been a concern because we might want to make the gap bigger, then the far must follow. Once gap is set, thickness tolerances set overall length.
Collar is another concern, as it needs to be shimmed during assembly to keep it straight.
Need enough tolerance in center to deal with this. During assembly of 4PP, there were crunchy moments when the scintillator contacted, probably the collar. Recommendation is to increase the bypass tolerance. The bypass tolerance is near the point where another whole strip will be sacrificed if it is increased. A long discussion of the sag in the columns and the collar shimming procedure showed people still think this is a major worry.