I see your point and agree that in a structural situation this might be an issue to consider.I think the problem with rivnuts in fiberglass is that rivnuts depend on the material they’re in not compressing. In fiberglass, the mushroom that forms on the back of the rivnut can crush the fiberglass and deform it. A washer would improve that situation, but if you can get a washer on there, you can get a nut on there.
I agree with all said, stay away from sheet metal screws in fiberglass.it is not about strength to hold on a plastic cover but make sure you counter sink the gelcoat to help prevent the gelcoat from cracking. tapping is better because it cuts the treading, removes the material and the sheet metal screw cracks and displaces the material which almost always cracks the gelcoat.
Glad you followed-up.Just a follow up on this.
I threaded 6 holes in glass a bit over 1/4” thick and it worked like a charm. I frequently backed out the tap and cleaned it and the hole using a can of compressed air. The threads really held. I was a little surprised it works so well and I’m surprised that manufacturers use sheet metal screws because they completely suck.
Spot face, counter bore or dimple enough to submerge the top flange.That flange at the top limits mounting options
a standard helicoil is more versatile