Bought new stainless niche retaining ring (8-hole American Pentair) and the gasket. After getting 5 of the screws in tight, the last 3 screws won't grab the threads of the holes in the niche. Unfortunately, the 3 holes are together, so the ring and gasket won't seal tight. The screws go in, but when they get to the ring, they just keep turning. Screws are 1/4" 20, machine, flat-head. Tried plumbers tape, no good. Tried 2 sizes of plastic wall anchors but they didn't grab and just spun when screws were inserted. I'm tempted to just tap the holes with 5/16" 18 tap to get new threads and then use 5/16 screws. But I'm a little concerned about the filings falling down and potentially cutting the liner. Also, if I vacuum them out, will they rip the filter grids? And if I do the tapping, do I need to drain the water down below the holes?
Got any ideas? Thanks.