It’s standard 19mm slim pad.
The pad popping out seems to be a combination of two things:
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I don’t know what it is, but the finish feels almost… slick… and since silicone isn’t actually an adhesive in the strict sense of the word, I wonder if there was never a really mechanically-sound bond being made
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Joey did “OK” with his silicone… but it could’ve been better.
Once the response was out, I could see that there were quite a few small voids. If it had been seamlessly edge-to-edge, it possibly wouldn’t have popped out.
I have some Flow Groove pads on the way which I think I’ll put in rather than applying my own gasket maker. Tempted to try to prep the surface of the groove a bit first, for better adhesion.