I just saw thougth I'd mention this.You say that what was in there for a plug was what they use in a boat[was that the expandable plug] What you better do before ordering a plug take a look into the hole where the plug goes and make sure that the threads haven't been stripped out.Could be why they used a plug like that instead of the proper one.If the threads are stripped out then your going to have to either buy a bigger size self tapping plug and thread it into the hole.{these plugs are avaliable at say Napa.} just make sure you put it in straight so it seals properly.Of course you can buy a kit thats a helicoil meaning you use the tap that comes with the kit and then screw in a helicoil making new threads for a new plug.Or you can use a biggier size tap to make new threads and put in a larger size plug.Its not hard at all to do.The first idea I mentioned is the easiest,this was a common fix for many a oil pan at one time.Larry
