Heater Flapper Stud Size and more???

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I have read up on the broken heater flapper stud solutions. I don't weld and 5 of 6 of mine are broken. I am looking at the Rivet nut solution. Can anyone tell me the sixe of the threaded studs in metric or standard? Also once I drill out the old ones what is the thickness of the metal at the site of those studs? I have located some stainless steel rivet nuts but I need this info to insure I order the correct size. If this goes well I will post some pictures?