On Jul 27, 7:31*pm, "hr(bob) "
wrote:
...snip....
You might have to neck down the 8 in to 7 in and use the 7 in cap.
Have you tried *a large-scale hardware place like Grainger?

no room for the neck down,
again the elbow turns, and inserts slightly INTO the wall; the wall
cap is thick so that its edge is almost flush with the interior wall.

that means that *IF* the wall cap were 8 inch; I'd be done