I had my in-ground gunnite pool "rebuilt" back in 2005. The work included installing a new 2" return line with 3 eyeball return fittings. I was trying to find a way of temporarily restricting my return flow (to add head...see my post in Pumps) by inserting a plastic disc inside the eyeball retainer and screwing it back on. When I did this on the first outlet (closest to the pump), the pump blew the fitting completely out of the return line. It was a slip fitting and I thought it would have been cemented in to the 1-1/2 branch pipe.
Is it normal practice to simply friction fit these into the return lines or did the pool company neglect to do this?
Instead I used the disc on the outlet at the far end of the return line and it held fast there, although I did not get the same pressure increase as I did by manually blocking the first outlet.