Clean Out Plugs

[Ld056]

Does anyone know where to get the proper plugs for a Flathead Mercury V8, with the 5/8" clean out holes in the crankshaft ? I have Dorman Number 555-010 Plugs, but I don't see HOW they could possibly fit since they are slightly larger than the 5/8" hole ! I've ruined 2 plugs already. Any help appreciated. Also, I've been on line, checked with about a dozen vendors, and no one seems to habe this simple little part.