IMHO, allthread is evil and should NEVER be used for air cleaner studs!!!! It isn't hardened at all........ Unless it was in the previous build, having it on the carb for only 4K miles should tell you how inadequate this material is to use in this application.....as a carb stud!
If this 3" piece is still in an intake port/valve you should be able to see it with one of those camera on a snake deals.....go in thru intake ports and or spark plug holes.....but with antifreeze in your oil.....your future task list seem obvious! Just a matter of which head comes off first......
BUMMER BIG TIME! Hope it turns out to be salvageable!
Steve