I've bought these threaded and O ringed caps and bungs for oil fill caps in valve covers. They're not hard to find.
Like many things it's mostly in the way you "search" for things.... I find I have to search and then refine my search as I see the way things are worded etc.
Can't get the link to post correctly -- so maybe copy and paste it instead of just clicking on it. Should take you to a page with all kinds of weld in bungs with screw
in caps
http://www.racepartsolutions.com/ind...?custom_f_2643[0]=57656c642d6f6e2043617020262042756e67&Itemid=564[/U]