I did mention a few suppliers and we don't want to favor one over the other. But here is a screen capture of one of the belly pan rivets from one of those sites. (Thank you Ahzmyrah; this is stated to be 3/16" diameter and a grip length of 3/8").
Note the large flange diameter. Not only do the thin belly skins get thinner and more brittle over time, Jim is correctly noting that the hole in the belly skin corrodes wider & faster. (bimetallism?)
Originally Posted by BIGED52
Jim, Bob forgot to tell you to use some vulkem to seal each hole and coat each rivet with it before you install them. That way you are protected for water until you do a more permanent repair. Ed
This won't hurt and indeed is indicated for any rivets replaced above the rubrail. It may not be necessary for belly rivets but at least might keep water away from the steel frame where the reaction is occurring -- don't need to enhance the frame holes corroding. Just don't use silly-cone!
The belly skin laps should be unsealed because ventilation is necessary. So the big picture is ya don't have to get overly fanatical to seal anything at all on the belly. The point of sealing belly rivets is comme ci, comme ça